The present invention relates to a method and a system of inserting a watermark for H.264/AVC, and provides a method which includes extracting Sign of Trailing Ones for one or more 4x4 blocks included in each video sequence of the compressed bitstream, making the Sign of Trailing Ones extracted for the one or more 4x4 blocks correspond to one or watermarks in order, and manipulating each of the Sign of Trailing Ones according to a corresponding watermark bit. Furthermore, the present invention provides a system performing the method. The present invention can easily applied to a real-time watermarking of contents for broadcasting because of not changing a video bit rater after watermarking. Moreover, a watermarking scheme of the present invention can add a watermark module very simply without a separate complicated circuit in a video coding device.