We report the observation of room temperature lasing action in optically pumped GaN nanopillars. The nanopillars were fabricated by patterned etching and crystalline regrowth from a GaN substrate. When nanopillars were optically excited, a narrow emission peak emerged from the broad spontaneous emission background. The increasing rate is nine times faster than that of the spontaneous emission background, showing the onset of lasing action. The lasing occurs right at the center of spontaneous emission rather than the often reported redshifted wavelength. A spectroscopic ellipsometry analysis indicates that the gain of lasing action is provided by exciton transition.