1. A method for a moving picture coding system to derive at least one motion vector of a bi-predictive block in a current picture from a motion vector of a first block in a first picture, comprising: selecting, by the moving picture coding system, a list 0 motion vector of the first block in the first picture as the motion vector for deriving list 0 and list 1 motion vectors of the bi-predictive block if the first block has both a list 1 motion vector and the list 0 motion vector, the first picture being permitted to be located temporally before the current picture and permitted to be located temporally after the current picture; deriving a first temporal distance between the current picture and a reference picture of the current picture; deriving a second temporal distance between the first reference picture and a reference picture of the first picture; scaling the selected motion vector based on the first and the second temporal distances; and deriving the at least one motion vector of the bi-predictive block by applying a bit operation to the scaled motion vector, the bit operation including 8 bits right shift.