1. A method of deriving motion vectors of a bi-predictive block based on a list 1 motion vector of a co-located block in a reference picture performed by a moving picture coding system, the method comprising: selecting, by the moving picture coding system, the list 1 motion vector of the co-located block in a first reference picture as the motion vector for deriving the motion vectors of the bi-predictive block, if the co-located block only has the list 1 motion vector, the first reference picture being a type of reference picture permitted to be located temporally before or after a current picture including the bi-predictive block; scaling, by the moving picture coding system, the selected motion vector based on temporal distance between the current picture and a second reference picture referred by the co-located block; and deriving, by the moving picture coding system, the motion vectors of the bi-predictive block based on the scaled motion vector.