The results of photoluminescence investigation of nominally undoped bulk InP at 2K are reported in this paper.The emissions near the band-gap are carefully analyzed. It was found that the intensity of the peak due to exciton bound to the neutral donor (D~0, X) decreases while the intensity of the peak due to the exciton bound to the neutral acceptor (A~0, X) increases with increasing excitation power and its mechanism was pro- posed. Mg and Zn were identified as residual acceptor impurities in the material. The ionization energy calculated for Mg acceptor is 41.5 meV.