Thin-film CdS/CdTe solar cells fabricated on glass substrates have been considered as one of the most promising candidates for large-scale applications in the field of photovoltaic energy conversion. The recorded experimental efficiency of these cells is about 16%–17% and the corresponding theoretical values are more than 28%. The main causes of efficiency loss are due to optical and recombination losses. Most of the theoretical literatures either study the effect of recombination or optical losses on the CdS/CdTe solar cell efficiency. The present work studies the effect of both the optical and recombination losses on the current density and hence the solar cell efficiency. Calculations of optical losses have been carried out based on the optical constants (refractive index and extinction coefficient) of materials used and the thickness of ITO and CdS layers. Calculation of recombination losses has been based on the values of width of space-charge region and the absorption coefficient for CdTe. It has been found that the reflection losses of the interfaces air-glass, glass-ITO, ITO-CdS, and CdS-CdTe decrease the short-circuit current (JSC) from 31.24 to 28.2 mA/cm2 (9%). The absorption losses in ITO and CdS layers decrease JSC to 22.2 (20%). The recombination losses decrease JSC to 19.7 mA/cm2 (8%). The optical and recombination losses yield efficiency of CdS/CdTe solar cells in the range of 12%–16% at thickness 100nm of each layer of ITO and CdS. According to these results, there is a good agreement etween experimental and theoretical studies and this is the real start to develop the solar cells efficiency in the future studies