The multijunction solar cells performances study is essential for the design of the high-concentration photovoltaic. These cells can operate over a wide range of the incident radiation flux and a large temperature range. These two parameters (concentration and temperature) degrade the cell and require a cooling system. In this article, we have studied numerically the cooling temperature of InGaP/lnGaAs/Ge subcells under the concentrated illumination. For this, we have presented the performance of each subcell as a function of the temperature and concentration sunlight. The different high concentrations ratios (1, 10, 100, and 1000 sun) have been conducted according to the dish-style concentration photovoltaic system for three temperature values T = 300, 500, and 800 K. The results show that under high concentrated light intensity conversion, the performances of these three subcells (efficiency, open-circuit voltage, short-circuit current, and fill factor) were decreased with increasing the temperature. The main objective of this study is to find the limit temperature of each subcell in order to introduce the cooling system. Thus, we can avoid the degradation of the tandem solar cell under the concentrated illumination.