This paper presents a hybrid de-interlacing algorithm that converts video from interlaced format to progressive format. This hybrid algorithm effectively combines two existing de-interlacing techniques: one that offers high vertical resolution and the other that is robust to erroneous motion vectors. The combination is based on a new measurement of motion vector reliability, so that the hybrid algorithm is dominated by the high-resolution technique if motion vectors are reliable; otherwise, it approaches the robust technique. Motion vector reliability is measured using the a posteriori probability of motion vectors. Experimental results show that this is an effective measure of motion vector reliability. The hybrid algorithm offers high spatial resolution without artifacts caused by erroneous motion vectors, and outperforms four of the best existing techniques in terms of peak signal-to-noise ratio.