In one embodiment, 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 includes selecting, by the moving picture coding system, a list 1 motion vector of the first block in the first picture as a motion vector for deriving list 0 and list 1 motion vectors of the bi-predictive block if the first block only has the list 1 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, scaling the selected motion vector and deriving the list 0 and list 1 motion vectors of the bi-predictive block by applying a bit operation to the scaled motion vector, the bit operation including 8 bits right shift.