In this paper, we propose a key-based video watermarking system in which the watermark embedding and the video encoding are processed at the same time on MPEG-2. Since the watermark information would propagate to inter-frames through the motion compensated coding, the watermark is embedded in a single intra-frame but can be extracted from all frames in the same group of pictures (GOP). The watermark is embedded in the low frequency DCT coefficients of the intra-frames based on the block polarity. The block polarity is Tri-state Exclusive-Or (TXOR) with the watermark to generate the secret key, which labels the block locations of the embedded watermark. In the decoding end, the block polarity over a GOP is calculated by a weighted voting procedure according to the frame weighting. Finally, the watermark over a GOP can be obtained by TXOR operation of the key and the block polarity. The simulation results show that the system has great imperceptibility that the PSNRs of the watermarked frames are almost the same as the un-watermarked ones and more accurate normalized correlation (NC) can be obtained as well.