As filed with the U.S. Securities and Exchange Commission on December 19, 2013 File Nos. 333-179904 and 811-22649 UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM N-1A REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933 x Post-Effective Amendment No. 40 x and/or REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY ACT OF 1940 x Amendment No.40 x (Check appropriate box or boxes) iShares U.S. ETF Trust (Exact Name of Registrant as Specified in Charter) c/o State Street Bankand Trust Company 200 Clarendon Street Boston, MA 02116 (Address of Principal Executive Office)(Zip Code) Registrant’s Telephone Number, including Area Code: 989.654.5113 The Corporation Trust Company 1209 Orange Street Wilmington, DE 19801 (Name and Address of Agent for Service) With Copies to: MARGERY K. NEALE, ESQ. BENJAMIN J. HASKIN, ESQ. EDWARD BAER, ESQ. WILLKIE FARR& GALLAGHER LLP WILLKIE FARR& GALLAGHER LLP BLACKROCK FUND ADVISORS 1, N.W. NEW YORK, NY 10019-6099 WASHINGTON, D.C. 20006-1238 SAN FRANCISCO, CA 94105 It is proposed that this filing will become effective (check appropriate box): ¨ Immediately upon filing pursuant to paragraph (b) x On January 17, 2014, pursuant to paragraph (b) ¨60 days after filing pursuant to paragraph (a)(1) ¨ On (date) pursuant to paragraph (a)(1) ¨75 days after filing pursuant to paragraph (a)(2) ¨ On (date) pursuant to paragraph (a)(2) If appropriate, check the following box: x The post-effective amendment designates a new effective date for a previously filed post-effective amendment. Explanatory Note This Post-Effective Amendment No. 40 to the Registrant’s Registration Statement on Form N-1A is being filed pursuant to paragraph (b)(1)(iii) of Rule 485 of the Securities Act of 1933, as amended (the “1933 Act”), solely for the purpose of delaying, until January 17, 2014, the effectiveness of the registration statement for the iShares Core Allocation Conservative ETF, iShares Core Allocation Growth ETF, iShares Core Allocation Moderate ETF and iShares Core Allocation Moderate Growth ETF (the “Funds), filed in Post-Effective Amendment No. 20 on September 9, 2013, pursuant to paragraph (a) of Rule 485 of the 1933 Act. The effectiveness of the Registration Statement of the Funds was previously delayed pursuant to paragraph (b)(1)(iii) of Rule 485 of the 1933 Act as follows: PEA No. Date Filed Automatic Effective Date 29 November 22, 2013 December 20, 2013 This Post-Effective Amendment No. 40 incorporates by reference the information contained in Parts A, B and C of Post-Effective Amendment No. 20. SIGNATURES Pursuant to the requirements of the Securities Act of 1933, as amended, and the Investment Company Act of 1940, as amended, the Registrant certifies that it meets all the requirements for the effectiveness of this Registration Statement pursuant to Rule 485(b)(1)(iii) of the Securities Act of 1933, as amended, and has duly caused this Post-Effective Amendment No. 40 to the Registration Statement to be signed on its behalf by the undersigned, duly authorized, in the City of San Francisco and the State of California on the 19th day of December, 2013. iSHARES U.S. ETF TRUST By: Michael Latham* President and Trustee Date: December 19, 2013 Pursuant to the requirements of the Securities Act of 1933, as amended, this Post-Effective Amendment No. 40 to the Registration Statement has been signed below by the following persons in the capacities and on the date indicated. By: Michael Latham* President and Trustee Date: December 19, 2013 John E. Martinez* Trustee Date: December 19, 2013 George G.C. Parker* Trustee Date: December 19, 2013 Cecilia H. Herbert* Trustee Date: December 19, 2013 Charles A. Hurty* Trustee Date: December 19, 2013 John E. Kerrigan* Trustee Date: December 19, 2013 Robert H. Silver* Trustee Date: December 19, 2013 Robert S. Kapito* Trustee Date: December 19, 2013 Madhav V. Rajan* Trustee Date: December 19, 2013 /s/ Jack Gee Jack Gee Treasurer Date: December 19, 2013 * By: /s/ Jack Gee Jack Gee Attorney-in-fact Date: December 19, 2013 * Powers of Attorney, each dated June 11, 2013, for Michael Latham, Charles A. Hurty, Cecilia H. Herbert, John E. Kerrigan, Robert H. Silver, George G.C. Parker, John E. Martinez, Madhav V. Rajan and Robert S. Kapito are incorporated herein by reference to Post-Effective Amendment No. 17, filed August 22, 2013.

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