N,N,N- trimethyl chitosan (TMC) with a quaternization degree of 40％ was used as a vectorfor siRNA delivery. Nano-sized complexes were formed in water by mixing siRNA with TMCsolution under vortex, followed by incubation for a period of time. The majority of particlesshowed spherical morphology under TEM with a diameter less than 100 nm. Generally, largerparticles were formed under higher temperature and longer incubation. The smallest particlesize appeared at an N/P ratio of 10. The complexes showed positive surface charge, whosevalue increased along with the increase of N/P ratio, and leveled off at +20mV above the N/Pratio of 10. When the N/P ratio was larger than 10, the binding efficiency of TMC with siRNAwas higher than 90％. Under the treatment of 25％ FBS, the complexed siRNAs with N/Pratios of 10 and 20 were intact for up to 12h and 48h, respectively. TMC/siRNA complexeswith an N/P ratio larger than 5 could efficiently enter into HEK 293 cells, which were trappedinitially in lysosomes but could escape and locate in cytoplasm. Gene silencing property wastested by using enhanced green fluorescent protein (EGFP) as the targeted gene. EGFPexpression was reduced to approximately 60％ by the complexes with an N/P ratio of 10 or 20.At a still higher N/P ratio, the gene silencing efficiency was comparable with that ofLipofectamineTM 2000. Moreover, specific silencing was confirmed by the performance of adose-dependency and non silencing effect of sequence-mismatch siRNA. Meanwhile, nosignificant cytotoxicity was detected for the TMC/siRNA complexes. This study suggests thatTMC is an effective vector for siRNA delivery.