Abstract: Two-dimensional unsteady numerical simulation was carried out to inve s tigate the flowfields surrouding a synthetic jet actuator and its effect on the mixing in coaxial jets. The synthetic jet flowfield and the coaxial jets flowfie ld were investigated. The results show synthetic jet comprises a series of vorte x pairs, which distribute around the centerline symmetrically. Its frequency kee p same to the actuator′s frequency. This is a disturbed force for the outer flo w field. The movement velocity of vortex pairs is nearly constant. The formation a nd development of the outer flowfield have the same rule as that of the inner fl owfield. But they are opposite to each other at the certain moment. As a result of actuating device, the centerline velocity distribution shows that the amplitu de decreases gradually, which represents the radical characteristic of vorticity d evelopment. The mixing ratio of coaxial jets increases comparatively because of the synthetic jet actuators, and the mixing effects in coaxial jets also become more agreeable.