Abstract: Microwave irradiation was employed to demulsify the water-in-oil (W/O) emulsions which encountered in refinery industry. Study of optimization of percentage water separated in microwave irradiation is crucial to obtain the cost effective. In this study, three types of crude oils consisted of heavy and light crude oil are used. The optimal conditions for microwave irradiation were determined by Response Surface Methodology (RSM) for each crude oil. Correlation analysis of the mathematical regression models indicated that quadratic model could be employed to optimise the microwave irradiation in each crude oil. Each crude oil could be demulsified in microwave irradiation for crude oil B and microwave-assisted chemical for crude oil A and C within 2.40 to 2.56 min for 38 to 64.10% of water separated. Thus, microwave irradiation can be an alternative method in demulsification by cost effective within short time processing.