Austenitic stainless steel (SUS304L) specimens were treated by argon ion bombardment, and then were bonded by diffusion bonding method. The effect of argon ion bombardment treatment on joints was investigated by tensile strength tests, and the cause of improvement of mechanical properties was analyzed by microstructure observations of the bonded interface and the fractured surface. The results showed that the tensile strength of SUS304L stainless steel joints was improved, and reduction of area was increased by argon ion bombardment treatment. At the interface of the joints at 900degreesC, the original bonding line disappeared. The fractured surface was ductile, and the amount of inclusions was very little. The cause of tensile properties improvement was found to be the decrease in inclusions at the bonded interface. The remainder of small amount of inclusions at the joints with ion bombardment was considered to be contaminated again after argon ion sputtercleaning.