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 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; and deriving the list 0 and list 1 motion vectors of the bi-predictive block by applying a bit operation to the selected motion vector.