1. A motion prediction method, comprising: (a) obtaining a plurality of motion vector predictors to be included in a candidate set for motion prediction of a current unit of a current frame; (b) determining whether the current frame is a non-reference frame which is not referred to by other frames for motion prediction; (c) when the current frame is not the non-reference frame, removing any motion vector predictor corresponding to a previously coded frame from the candidate set; and (d) predicting a motion vector of the current unit according to the motion vector predictors of the candidate set.