A method and device of identifying payload of a data packet in a TCP stream. The method includes the steps of: calculating a payload signature according to information in header of the data packet in the TCP stream; comparing the payload signature with a pre-stored file signature; determining the payload of the data packet in the TCP stream as belonging to a file corresponding to the pre-stored file signature, in response to a match between payload signatures of multiple data packets and the pre-stored file signature. The present invention can monitor and identify TCP streams by using a more efficient and lower cost solution.