High efficiency inverted metamorphic (IMM) GaInP/GaAs/In0.3Ga0.7As(1.0 eV) triple-junction solar cells have been fabricated by growing In0.3Ga0.7As(1.0 eV) sub-cell using step-graded buffer layer, which is 2% lattice mismatch to the GaAs middle cell. The high crystalline quality and low threading dislocation density are confirmed by high resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM). The quantum efficiency and I-V characteristic are measured for the IMM GaInP/GaAs/In0.3Ga0.7As solar cells, as well as for the conventional triple-junction solar cell based on Ge substrate (GaInP/GaAs/Ge). The efficiency of the designed cell with an area of 10.922 cm2 is 32.64% (AM0, 25 ℃), which is 3% higher than the conventional GaInP/GaAs/Ge triple junction solar cell.