Under the critical thermal conditions, the human body cannot adapt itself to the environment by using physiological thermoregulatory mechanisms. Under these conditions, using the protective clothing is one of the effective ways to protect the human body against the thermal injuries. Therefore, in the present study, the effect of using Phase Change Materials (PCMs) on the performance of firefighters’ protective clothing has been numerically investigated under the critical scorching conditions. The main contribution of this study is the simultaneous modeling of a PCM based protective clothing with physical and physiological mechanisms of the human body. For this purpose, a multi-layer protective clothing with a PCM layer has been considered and its thermal performance has been investigated under scorching conditions for three different arrangements of the layers. The results show that the middle layer of protective clothing is the best position for implementing the PCM. Also, and is the best melting temperature for the mentioned PCM is about 40˚C. Moreover, the results indicate that using the PCMs in protective clothing can increase the thermal tolerating time from 300 seconds (for non-PCM protective clothing) up to 900 seconds, under the scorching conditions.