A numerical simulation is performed to study the far-field diffraction properties of planar diamond waveguides. The far-field intensity distributions of a planar air waveguide and a diamond waveguide with different distances are given by numerical calculations. In the experiment, the diffraction patterns on the screen with different distances are recorded using a He-Ne laser as the light source, wherein the laser beam is coupled with and propagates in the diamond waveguide. The simulation results are found to be consistent with the experimental ones.