The effects of GaAs substrate off-orientation on GaInP/AlGaInP heterojunction solar cells were investigated. The performances of solar cells fabricated on 2° and 10° off GaAs substrates were compared. The short circuit current densities were 10.44 mA/cm2 for the 10° off sample, 7.15 mA/cm2 and 7.41 mA/cm2 for the 2° off samples, which showed 30% higher short-circuit current density for 10o off samples. Also, 30% higher external quantum efficiencies and smooth surface morphology were observed in the solar cell fabricated on the 10° off GaAs substrate. Secondary ion mass spectrometry depth profiles showed that the solar cells on 2° off substrates had a 20-times higher oxygen concentration than the solar cells on 10o off GaAs substrate in the n-GaAs/GaAs buffer layer. The 30% reduction for the solar cells on 2° substrates in short circuit current density (Jsc) was attributed to the higher oxygen concentration of the 2° off samples than the 10° off samples. I-V characteristics comparison between different front contact grid patterns was also performed for optimization of grid contacts. A 0.47 V bandgap-voltage offset, one of the device performance figures of merit to compare PV cells with different materials, was obtained.