Protection of digital assets from piracy has received increasing interests where sensitive, valuable data need to be released. This paper addresses the problem of watermarking spatial trajectory database. The formal definition of the problem is given and the potential attacks are analyzed. Then a novel watermarking method is proposed, which embed the watermark information by introducing a small error to the trajectory shape rather than certain data values. Experimental results justify the usefulness of the proposed method, and give some empirical conclusions on the parameter settings.