Microwave (MW)‐assisted de‐emulsification has been attractive in processes of petroleum production and refining. However, the mechanism and optimal operation remain poorly understood. In this study, the behaviour of decane‐in‐water emulsion under MW irradiation was monitored in‐situ through a size measurement system equipped with the reactor and surface tension profiles. The results indicated that the bubble was formed around the oil droplet during MW irradiation. The MW‐induced bubbles can enhance de‐emulsification in a similar mechanism as the flotation column. The efficiency of MW de‐emulsification depends on emulsion content and MW power. Although a higher MW power is more effective, boiling caused by excessive energy must be considered due to the higher local heating. As a result, moderate power is more desirable. For the decane‐in‐water emulsions in this study, the optimal condition was determined to be around 500 W.