The low temperature synthesis of Cr3+ doped MgAl2O4 nano- crystalline spinels was developed. Their structure and morphology were investigated. The average grain size of nanocrystals increased with increasing temperature of annealing process. The smallest nanocrystals of 9 nm diameter were obtained at 700 °C. The luminescence properties of Cr3+:MgAl2O4 nanocrystals were investigated at room and liquid nitrogen temperatures. It was found that the luminescence, excitation spectra and lifetimes were strongly affected by grain size of nanocrystals. Following the excitation spectra it can be concluded that with increasing the grain size a contribution of 4T1 band to the overall phosphorescence increased. It was also noted that with increasing the grain size the spinels become less disordered what appeared in increasing the 00 ? 0 phosphorescence. The phosphorescence decay profiles become exponential and the effective decay times longer.