Photonic crystals have a special dispersion characteristic, which can lead to negative refraction. The negative refraction of a two-dimensional photonic crystal slab is calculated theoretically and measured experimentally with the beam-displacement method. The two-dimensional field distribution for an incident light refracted by this photonic crystal slab is obtained. The measured result shows that, as for the transverse electric (TE) light, the negative refraction in photonic crystals and the horizontal displacement in the opposite direction for the light exiting along the exit surface occur, and thus the effective refractive index is calculated as -0.44. In contrast, as for a transverse magnetic (TM) light, the negative refraction does not happen at all. The experimental result is well consistent with the simulated one.