Surface cannot be created with nurbs curves by pressing F key I followed several tutorial to create a nurbs curve, and duplicate it in edit mode. Of course I pressed A key to select all points and F key to create surface. But there was no surface be created, a start point of one curve and an end point of another point were connected instead. 



Is there anybody get any idea about this case?
You simply need to use one of the Nurbs Surface types. Add > Surface > Nurbs Curve


A Curve, whether a nurbs or bezier is only a stroke. It can not have volume, it is only ever a line.
A Surface is a full 3D solid that can have volume. All the expected commands will work: Fill F, Extrude E etc.