Scattering of electromagnetic waves from periodic surfaces is considered in time-domain for an oblique angle of incidence. The finite-difference time-domain (FDTD) method is used to obtain numerical solutions without resorting to frequency-domain analysis and Fourier transformation. For the application of FDTD method to the oblique incidence case, Maxwell's equations are transformed such that the computational domain can be truncated by using periodic boundary conditions. The FDTD method is then used to solve the transformed equations. In solving the transformed equations by the FDTD method, the absorbing boundary conditions are modified and the eigenvalues of the system are determined for the stability analysis. The final results are obtained by using the inverse transformation. Since the transformation is very simple, the computational time is primarily determined by the FDTD solution of the transformed equations. The theoretical results are illustrated by calculating the scattered fields in the computational domain as a function of time for a pulse incident at an oblique angle. The time domain results are also transformed into frequency domain and compared with the results obtained using a standard frequency domain technique.