We propose a derivation scheme of a motion vector for interlace format. The adjustment of the vertical component of a chroma MV for 4:2:0 format and the adjustment of the vertical component of a luma prediction MV are introduced in order to remove inconsistency between the current derivation process of motion vector predictor and the nature of interlace format in HM4.0. The proposed scheme achieves up to 1.0dB PSNR improvements in chroma components and up to 3.4% bit-rate reduction.