Orientia tsutsugamushi, the causative agent of scrub typhus, is an obligate intracellular pathogen. Recent studies show that the complete genome sequence of Orientia tsutsugamushi have been determined. However, the early signaling events involved in the entry of O.tsutsugamushi into mammalian cells remains a challenge. In this study, we demonstrate that adherence ability and comparison of three major outer membrane protein TSA56, TSA47 and TSA22 of O.tsutsugamushi. Through expression and purification of three type-specific antigen 56-kDa (include TSA56-antigen domain I, TSA56-antigen domain III), 47-kDa and 22-kDa of O. tsutsugamushi , antiserum immunoblots from 22 clinical O. tsutsugamushi-infected patients and in vitro adhesion assay of E.coli overexpression outer membrane protein of O. tsutsugamushi , the antiserum titer and adherence ability of bacterial outer membrane proteins are determined. The data show that antiserum titer against three major outer membrane proteins of O. tsutsugamushi was markedly higher in TSA56 compared to TSA47 and TSA22. In adhesion assay, adhesion of host cells by TSA56 was readily than TSA47 and TSA22. Furthermore, adhesion experiment and antiserum titer against antigen-domain I (ADI) region (19-114 aa) in the extracellular domain of TSA56 was also significantly higher than previously reported antigen-domain III(ADIII) region (237-366 aa) which facilitates the invasion of O. tsutsugamushi through interaction with fibronectin .Taken together, these results clearly indicate that O. tsutsugamushi exploits TSA56-mediated bacterial adhesion, abundant antiserum titer and ADI region of TSA56 may draw another adhesion site (except for previously reported ADIII) to invade eukaryotic host cells.