1. A method of deriving at least one motion vector of a bi-predictive block based on a list 1 motion vector of a co-located block in a reference picture using a bit operation performed by a moving picture coding system, the method comprising: selecting, by the moving picture coding system, a list 1 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, if the co-located block only has the list 1 motion vector, the reference picture being a type of reference picture permitted to be located temporally before a current picture including the bi-predictive block when the reference picture precedes the current picture and permitted to be located temporally after the current picture when the reference picture follows the current picture; and deriving, by the moving picture coding system, at least one motion vector of the bi-predictive block by applying a bit operation to the selected motion vector.