In this paper, we introduce a kind of watermarking system for IP protection (IPP). The copyright is encrypted and then embedded into the design as the watermark in buffer insertion stage. This watermarking technique can identify the design copyright uniquely, and is fit for both ASIC using standard cells and full-custom design. We have evaluated the technique on several testing designs, the inserted results show that the watermarking process achieves 100% success causing little overhead on design performance. Extraction and identification process of the watermark is also analyzed. The watermark embedded is hard to be found out and tampered away, and the technique can be integrated into EDA tools for manufacturing.