The grain-size effect on the phase transition induced by pressure in PbS was studied by in situ high-pressure electrical resistance and synchrotron radiation x-ray powder diffraction measurements. The mean transition pressure of the B1-to-B16 phase transformation was found to be 6.3±1.3 GPa in 8±1 nm PbS while it is 3.1±0.7 GPa for 10 µm PbS. The resistivity of the B16 PbS phase decreases exponentially with pressure in both samples at ambient temperature. They follow R[proportional]exp(–CP), where C = –0.64 GPa–1 for 10 µm PbS and C = –0.34 GPa–1 for 8±1 nm PbS. These results are discussed in terms of a decrease of energy band gap with increasing pressure. ©2000 American Institute of Physics.