Objective: To prepare celastrol-Coicis Semen oil microemulsions (CC-MEs) and evaluate its physicochemical properties and in vitro anticancer activity. Method: The optimal oil phase

emulsifier and co-emulsifier of CC-MEs were selected by examining solubility of celastrol in different media.According to determination of particle size

Zeta potential

morphology

encapsulation efficiency and drug loading

the prescription was optimized.The stability and in vitro release of the microemulsions was investigated.The cytotoxicity of CC-MEs on cervical cancer HeLa cell was studied by MTT assay and its in vitro anti-tumor activity was evaluated. Result: Optimum prescription was as following:celastrol of 10 mg

Coicis Semen oil of 400 mg as material and oil phase

cremophor RH40 of 450 mg and polyethylene glycol 400 (PEG-400) of 150 mg.CC-MEs displayed the spherical surface with mean particle size of (31.63±0.63) nm

polydispersity index (PDI) of 0.06±0.01 and Zeta potential of (-10.14±1.35) mV.The cumulative in vitro release rate of celastrol was (19.89±0.59)% in 48 h with a sustained-release characteristics.MTT experiment showed that 50%inhibitory concentrations (IC50) of CC-MEs were 0.82 μmol·L-1 counted as celastrol and 20.7 mg·L-1 counted as Coicis Semen oil;the combination index (CI) was 0.93

indicating that celastrol and Coicis Semen oil had a synergistic effect. Conclusion: CC-MEs presents small particle size

well-distributed

good stability and less excipients.In addition

preparations with the compatibility of celastrol and Coicis Semen oil can enhance the inhibitory effect on proliferation of HeLa cell.