We have grown GaN films on ZrB2 substrates at room temperature (RT) by using a pulsed-laser deposition technique. Reflection high-energy electron diffraction observations have revealed that GaN growth can occur in a layer-by-layer mode, even at RT, and that the surfaces of the films are atomically flat. We found that intermixing reactions at the GaN/ZrB2 heterointerfaces, which have been the most serious problem for this structure until now, are well suppressed in the case of RT growth. Electron backscattered diffraction measurements have revealed that the tilt angle and the twist angle of the RT GaN are 0.23° and 0.24°, respectively, even for film thicknesses as low as 20 nm. The fact that RT GaN exhibits quite high crystallinity from the early stages of film growth can be attributed to the small lattice mismatch of this system.