1. A method of deriving at least one motion vector of a bi-predictive block in a current picture from a motion vector of a co-located block in a reference picture, performed by a moving picture coding system, comprising: selecting, by the moving picture coding system, a list 0 motion vector of the co-located block in the reference picture as the motion vector for deriving the at least one motion vector of the bi-predictive block regardless of whether the co-located block has a list 1 motion vector if the co-located block has the list 0 motion vector, the reference picture being a type of reference picture permitted to be located temporally before or after the current picture; selecting, by the moving picture coding system, the list 1 motion vector of the co-located block in the reference picture if the co-located block only has the list 1 motion vector; and deriving, by the moving picture coding system, the at least one motion vector of the bi-predictive block based on the selected motion vector.