The current-voltage properties of CdS/CdTe heterojunction were first discussed under ideal conditions (without any losses). Conversion efficiency of CdS/CdTe heterojunction solar cells was then investigated under AM1.5 illumination. The results showed that the conversion efficiency of solar cells could be achieved up to 27%, which was in agreement with the results predicted previously. Meanwhile, the realistic conversion efficiency of CdS/CdTe heterojunction solar cells was about 23% after taking into consideration of influence of some material parameters. Given considerable progress that has been made in manufacturing CdS/CdTe solar cells in recent years as well as considering a rational theoretical analysis, the conversion efficiency of 23% is achievable in the near future.