We have investigated the structure and photoluminescence (PL) properties of Se nanocrystals (NCs) obtained by rapid thermal annealing of a-Se films on Si substrate. The size of Se NCs in a trigonal phase increases linearly with increasing temperature. Moreover, three PL peaks located at 1.4, 1.7 and 1.83 eV are observed, which are attributed to the emission of defects in amorphous Se, donor-acceprter pair (DAP) recombination at the interface of amorphous Se and Se NCs, and interband transition of Se crystals, respectively.