A finite-difference time-domain (FDTD) scheme in Cartesian coordinate system is developed to analyze the guided-wave properties of a class of two-dimensional (2D) photonic crystals formed by square or triangular arrays of metal posts. With the application of the periodic boundary condition, the computing domain can be restricted to a unit cell. A modified Yee's grid is introduced to calculate the dispersive characteristic in the case of triangular lattice. As examples, several classic structures are analyzed, numerical results are compared with the results from other methods, and the agreement is found to be very good. This method can also be extended to the three-dimensioned (3D) case.