A motion vector estimation method and an encoding mode determining method for converting an input moving picture image into a compressed moving picture image suitable for a desired frame rate by re-determining a motion vector and a motion-compensated block type when the frame rate of the input moving picture image needs to be reduced. A method of estimating a motion vector of a motion vector of a macroblock of a current frame in a transcoder includes determining whether the motion vector of the macroblock of the current frame is created with reference to a macroblock of a dropped frame and estimating the motion vector of the macroblock of the current frame by tracing back a reference frame until an undropped frame is found, if the motion vector of the macroblock of the current frame is created with reference to the macroblock of the dropped frame.