Morphology and optical properties of GaN nanowhiskers grown by molecular beam epitaxy (MBE) have been studied in correlation with growth parameters. It was shown that the growth parameters can be tuned such that uniform, well separated and high-quality nanowhiskers are obtained. Using an optimized ramp of Ga beam equivalent pressure (BEP) during the growth, the tapering or coalescence of nanowhiskers can be suppressed. By increasing the growth temperature the density of nanowhiskers is reduced, but the crystalline quality is improved as can be concluded from cathodoluminescence (CL) results.