image decoding method including : receiving an encoded bitstream including information of i and p frames ; and executing motion compensation by synthesizing a predicted image of a current frame using motion vector information included in the encoded bitstream and a previously - decoded reference image . the motion compensation includes calculating intensity values at points where no pixels actually exist by bilinear interpolation performable using a positive rounding method and a negative rounding method ; wherein the bilinear interpolation is performed using the rounding method specified by rounding method information included in the encoded bitstream of the current frame when the current frame is a p frame ; wherein the rounding method information is included in a header section of the coded information of the current frame ; and wherein the rounding method information is not received from the encoded bitstream of the current frame when the current frame is an i frame .