We have investigated the photoluminescence properties of GaN quantum dots with sub-micron lateral resolution by means of Near-Field Scanning Optical Microscopy (SNOM) operating in illumination mode. The analyzed sample consists of several stacked planes of GaN/AlN quantum dots grown by molecular beam epitaxy on Si(111) substrate. A clear correlation between the surface topography measured by Atomic Force Microscopy on uncapped samples and the SNOM luminescence maps has been found.