 Prostaglandin E1 PGE1 have been reported to prevent and relieve the severity of VOD by Gluckman et al.. Lipo-PGE1 is a transporter of PGE1, which is superior in concentrating PGE1 and acts for prolonged time. Because of the prolonged effective time, lipo-PGE1 acts comparable effects by 1418 of PGE1 dose. Empirically, pediatric HSCT centers adopt lipo-PGE1 in dose of 1 mcgkgday 0.042 mcgkghr, which is 17 of the dose recommended by Gluckman, et al. This prospective study will investigate the concentration of lipo-PGE1 with preventive lipo-PGE1 with empirical dose 1 mcgkgday. 