Microwave treatment is a newly developed technique to expel resin from pine wood by generating high internal steam pressure. To understand the mechanism of deresination process during the treatment, a mathematical model was developed and was used to predict its critical conditions. The model indicated that the critical pressure inside Masson pine wood was 1.17×105 Pa and its corresponding critical temperature was 104 °C in order to expel the resin mixture from the interior to the surfaces. The experiment results showed that during microwave deresination large quantities of the resin overflowed to the wood surfaces when the temperature of the pine wood was controlled in the range 104-112°C. The result observed during the microwave treatment matched well with the critical temperature predicted from the simple mathematical model. The validated model could provide some understanding and improvement of microwave deresination process.