Abstract: OBJECTIVE To optimize the optimal formulation composition of fennel oil microemulsion gel and to investigate its transdermal performance in vitro. METHODS The formulations of microemulsion and microemulsion gel were optimized by central composite design-response surface method, and the transdermal properties of the microemulsion and its gel were investigated by Franz diffusion pool method. RESULTS The optimal formula of microemulsion was oil phase:emulsifier and co-emulsifier(Tween-80:diethylene glycol monoethyl ether=2.5:1):water=20:27.6:52.4, and the microemulsion gel of fennel oil was prepared. The microemulsion of fennel oil had good fluidity and stability, the average particle size was (42.03±0.9)nm, and the polydispersion coefficient(PDI) was (0.154±0.005). The obtained microemulsion gel of fennel oil was uniform and delicate, easy to spread, and had a certain fluidity. It was an opalescent translucent semi-solid gel. The results of in vitro transdermal performance showed that microemulsion and microemulsion gel were prepared from fennel oil as oil phase, and the in vitro transdermal absorption of fennel oil was increased by 3 times and 4 times respectively. CONCLUSION This study lays a foundation for the preparation technology of fennel oil and provides a theoretical basis for the extensive application of volatile oil drugs.