The ferromagnetic and electronic structure for the Al_xGa_1-xP and Mn-doped AlGaP₂ was studied by using the self-consistent full-potential linear muffin-tin orbital method. The lattice parameters of un-doped Al_xGa_1-xP (x= 0.25, 0.5, and 0.75) were optimized. The band-structure and the density of states of Mn-doped AlGaP₂ with or without the vacancy were investigated in detail. The P-3p states at the Fermi level dominate rather than the other states. Thus a strong interaction between the Mn-3d and P-3p states is formed. The ferromagnetic ordering of dopant Mn with high magnetic moment is induced due to the (Mn-3d)-(P-3p)-(Mn-3d) hybridization, which is attributed by the partially filled P-3p bands. The holes are mediated with keeping their 3d-characters, therefore the ferromagnetic state is stabilized by this double-exchange mechanism.