First-principle calculations within the framework of density functional theory are employed to study the structural, electronic, and half-metallic ferromagnetic properties of In1−x (TM)xP (TM = Cr, Mn) at concentrations (x = 0.0625, 0.125, 0.25)of transition metal in zinc blende phase. The investigations of electronic and magnetic properties indicate that In1−xTMxP (TM = Cr, Mn) at x = 0.0625, 0.125, and 0.25 are half-metallic ferromagnets with 100 % magnetic spin polarization. On the one hand, the total magnetization is an integer Bohr magneton of 3 μB and 4 μB for In1−xCrxP and In1−xMnxP, respectively, which confirms the half-metallic feature of In1−xTMxP compounds. On the other hand, the densities of states of majority-spin states show that the large hybridization between 3p (P) and 3d (TM) partially filled states dominates the gap, which stabilizes the ferromagnetic state configuration associated with double-exchange mechanism. The band structures depict that half-metallic gap at x = 0.0625 is 0.404 eV for In1−xCrxP which is higher than 0.125 eV for In1−xMnxP. Therefore, the largest half-metallic gap in In1−xCrxP at low concentration x = 0.0625 reveals that Cr-doped InP seem to be a more potential candidate than that Mn-doped InP for spin injection applications in the field of spintronic devices.