Abstract: Multi-junction Solar Cells (SC) made from III-V compound semiconductors are still in the development phase. Here, we perform calculations for multi-junction cells: GaInP and GaAs based on crystalline silicon (With band-gaps, respectively 1.85, 1.42 and 1.1eV) in order to obtain the optimal thickness and efficiency for each junction under the AM1.5 solar radiation spectrum. The ideal photo-current density is around 9.2 mAcm–2 In order to reduce the problem of mismatch between cells and the tunnel junction costs and fabrication, we use a new structure of tandem solar cells in it we separates the cells in a good theoretical model. The expected conversion efficiency (η), under the AM1.5 spectrum (without tunnel junction), was determined to be around 46%, making this an attractive III-V compound tandem cell with crystalline silicon to be investigated in the near future.