In this study, Ga1-yInyP/Ga1-xInxAs monolithic dual junction (DJ) solar cells (SCs) were designed and optimized using 0.00, 0.01, 0.05 and 0.30 values of Indium (In) content (x) for 0.58, 0.58, 0.62 and 0.72 values of (In) content (y), respectively. The short-circuit current density (Jsc), open-circuit voltage (Voc) and conversion efficiency () of the four modeling SCs were numerically calculated. The band gaps were aligned to obtain the best performance of the cells by optimizing the content of (In) into layers in each cell. It was determined that the band gap of the cells decreased as increasing the (In) content both of (x) and (y), but the of the SCs reduced accordingly. To obtain high it was suggested that the modeling Ga1-yInyP/Ga1-xInxAs structure on GaAs substrate could be grown as inverted to realize low cost hybrid SCs or DJ thin film SCs on a flexible substrate. Furthermore, the band gap alignment effects the Jsc, Voc and of the SCs, as well as effect of the cell temperature on these parameters were also investigated. In addition, device performance of the SCs was also discussed under one-sun both AM1.5G and AM0 spectral conditions.