Effects of magnetic antidots on the transport properties of zigzag-edged graphene nanoribbons (ZGNRs) are investigated by spin-polarized first-principles calculations combined with a nonequilibrium Green's-function technique. Specifically, the effects of antidots (or holes) with regular shapes (rectangular and triangular) are studied. It is found that rectangular holes with a zero total spin S-0 and triangular holes with a finite spin S-0 cause different effects on the equilibrium conductance of ZGNRs. A rectangular hole with zigzag edges parallel to the ribbon edges blocks the transmission of the band edges of both the valence band and the conduction band from both the spin-up channel and the spin-down channel. Thus a much wider transmission gap than the pristine ZGNRs can be observed. However, a triangular hole with zigzag edges blocks transmission from only one spin channel in either the valence-band edge or the conduction-band edge. Thus the gap width in the total conductance is not affected in this case. The difference originates from the different energy shift of the valence band and conduction band relative to Fermi energy as a result of two effects: finite-size effect and spin splitting from the antidot-induced effective internal magnetic field.