We have investigated the initial stage of GaN epitaxial growth on 6H-SiC (0001) at low substrate temperatures by pulsed laser deposition (PLD). We found that GaN grows epitaxially even at room temperature (RT) on atomically flat 6H-SiC (0001) surfaces, which can be explained by the enhanced surface migration of film precursors due to the use of PLD and atomically flat substrates. In situ reflection high-energy election diffraction observations have revealed that GaN films grown at above 300 °C proceed in a three-dimensional mode, while those at RT proceed in a layer-by-layer growth mode with atomically flat terraces and steps. The step height turned out to be 1.5 nm, which is the same height as the steps on the SiC substrates. This result indicates that the step height on the SiC surface is retained as the GaN grows.