We study spin-polarized cation vacancies in wurtzite structure semiconductors (BeO, ZnO, ZnS, CdS, BN, AlN, GaN and GaP) by using first-principles calculations based on the density functional theory. We find that C3v geometries are the most stable and are spin-polarized. Two majority spin electrons occupying the defect E level lead to the magnetic moment of 2 μ B in the case of II-VI semiconductors. On the contrary, in the case of III-V semiconductors, three majority spin electrons occupying the defect E and A1 levels induce the magnetic moment of 3 μ B . The spin polarization of cation vacancies in oxides and nitrides are found to be stable compared with other cation vacancies in II-VI and III-V semiconductors, respectively. We clarify that the effect of the symmetry lowering from C3v to C s is small and thus confirm that the spin polarized C3v geometries are the most stable.