The band structure of a two-dimensional photonic crystal (PhC) formed by a triangular lattice of air columns in a gyromagnetic material is investigated in the case when an external dc magnetic field is applied to it. It is shown that large magnetic-field-induced (MFI) bandgap is obtainable by optimizing the parameters of the PhC. The interface between a PhC with MFI bandgap and air may support unidirectional or bidirectional propagating edge modes, or even no mode, closely depending on the boundary shape of the truncated PhC. The transmission property of one-way mode sustained by the gyromagnetic PhC boundary is discussed through numerical simulation.
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