1. A method of deriving a motion vector of a bi-predictive block based on a list 1 motion vector of a co-located block in a reference picture, comprising: selecting the list 1 motion vector of the co-located block in a first list 1 reference picture as the motion vector for deriving list 0 and list 1 motion vectors of the bi-predictive block, if the co-located block only has the list 1 motion vector, the first list 1 reference picture being a type of reference picture permitted to be located temporally before or after a current picture, and the selected list 1 motion vector pointing to a second list 1 reference picture which is a reference picture of the first list 1 reference picture, the current picture including the bi-predictive block; scaling the selected motion vector based on temporal distance between the current picture and the first list 1 reference picture; and deriving the motion vector of the bi-predictive block based on the scaled motion vector.