Nanocrystalline GaN was ammonothermally synthesized at 430 degrees C with CsNH2 as mineralizer. X-ray powder diffraction (XRD) analysis showed that the nanocrystalline GaN were of hexagonal structure. The average diameter of nanocrystalline GaN was about 4.5 nm according to Scherer's formula using the full-width at half-maximum (FWHM) of the (110) peak. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) observation confirmed the size distribution and phase state of the GaN nanocrystals. The calculated lattice parameters obtained both from XRD and High-Resolution Transmission Electron Microscopy (HRTEM) were a = 3.197 A, c = 5.195 A, which were larger than the highly crystallized GaN bulk crystal. Room temperature photoluminescence (PL) measurement exhibited a weak band-edge emission at about 390 nm (3.18 eV), indicating an obvious red-shift from the bandgap of bulk GaN (3.39 eV). A green luminescence (GL) emission located at 485 nm (2.56 eV) and a yellow luminescence (YL) defect-related emission peak centered at 553 nm (2.24 eV) were also observed. The advantage of using CsNH2 as mineralizer for growth GaN, the possible origins of the red-shift of bandgap, YL and GL band were discussed. The red-shift of the bandgap of the as-synthesized GaN nanocrystals was possibly attributed to the expansion of lattice parameters.