Cooling garments containing phase change materials are one of the practical techniques used for improving thermal comfort and worker’s performance in hot environments and under protective clothing. Such garments absorb the body's excessive heat and help the body reach thermal comfort situation by reducing the heat content of the body. The amount of heat absorbed by the phase change materials and the efficiency of the cooling garment can be affected by several parameters. This study uses a software modeling to investigate the effect of different factors such as ambient temperature, the use of protective clothing over the cooling garment, and the type of phase change material bags coating on the efficiency of the cooling garment. The results showed that temperature of the environment in which the cooling garment is used could largely affect the cooling efficiency of such garments. Increasing the ambient temperature from 30 to 40℃ can reduce the cooling efficiency of the garment up to 70%. Also, the use of cooling garment under protective clothing was simulated in this paper as a practical way to improve the thermal situation of the body. It was also shown that insulating the outer surfaces of phase change material bags against the surrounding environment could enhance the cooling power of the garment by 20 to 34% depending on the type of the coverings.