Due to rapid demand on the thermoelectric refrigerator, an investigation concerning the effect of water volume on a thermoelectric cooler box performance and its COP has been conducted. The aim of this study is to know the performance and COP of the cooler box with water volume variations. The conduction heat transfer rate flowing from the ambient to the cooler box space is discussed deeply as this type of heat transfer rate is seldom to be elucidated in the literature and it can be the dominant of the heat load when there is no water volume inside the cooler box. The cooler box size was 390 mm x 320 mm x 530 mm and the water volume variations employed were ranging from 0 to 4500 ml. The power used was of approximately 51.27 W. The results indicate that increasing the water volume raises the cooler box space temperature and COP but decreases the conduction heat transfer rate. At 0 ml water volume, the conduction heat transfer rate increases and it gets constant, while at higher water volume the COP decreases with the time.