Abstract: The secretion system of gram-negative bacteria is divided into six types, named T1SS to T6SS. Among them, T4SS can transfer DNA, proteins and ribonucleoproteins. T4SS is not only beneficial to bacteria evolution through the horizontal transfer of resistance or virulence genes but also related to the pathogenicity of bacteria by transferring effectors to host cell. This paper reviews the latest progress on classification, structure, biological function of T4SS and identification methods of the effector proteins based on protein interaction and bioinformatics.