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 lumi- nescence properties of Cr3+:MgAl2O4 nanocrystals were investi- gated 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 exci- tation spectra it can be concluded that with increasing the grain size a contribution of 4T1 band to the overall phosphorescence in- creased. 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 be- come exponential and the effective decay times longer.