The combination of a microchannel and a circular synthetic jet has been proven as a promising cooling technology for microelectronic devices. However, the flow control and heat transfer characteristics of a circular synthetic jet in a microchannel are still not clear. This work numerically investigates the interaction between a circular synthetic jet and a cross flow in a microchannel using an in-house solver based on the finite volume method and analyzes its impact on the heat transfer process to further understand the heat transfer enhancement induced by a circular synthetic jet in a microchannel. The effects of the synthetic jet Reynolds number (Resj = 0 to 324), dimensionless stroke length (L/H = 1.8432 to 92.16), and cross-flow Reynolds number (Rec = 188 to 470) are studied herein. The results show that the main body of the vortex structure generated by the circular impinging synthetic jet during the discharge stage is a hairpin vortex. During the downstream motion of the hairpin vortex, the vortex legs gradually stretch and evolve into a pair of large-scale longitudinal vortex. The heat transfer enhancement induced by a circular synthetic jet can be divided into two parts according to the difference in acting mechanism: an impinging region with severe heat transfer fluctuation and an entraining region with a relatively stable heat transfer performance. The parametric study demonstrates that the enhancements of the time-area-averaged Nusselt number and the total pressure drop are mainly affected by Resj, while the transient heat transfer performance is determined by L/H. Furthermore, the growth rates of the time-area-averaged Nusselt number and the total pressure drop are almost independent of Rec.