Influences of InGaAs/GaAsP quantum wells on the quantum efficiency of GaAs solar cells are investigated. In addion of extending the absorption spectrum from 890 nm to 1000 nm, introduction of quantum wells has important effects on the quantum efficiency below 890 nm. In the range of shorter wavelengths(<680 nm), the GaAs control cells have higher quantum efficiencies, while in the longer wavelengths (680-890 nm), the quantum well solar cells have higher quantum efficiencies. This phenomenon can be explained by the difference in the absorption coefficients of quantum well structure and GaAs materials.