A heavy pollution case in December 2006 in Lanzhou was simulated using the Nested Air Quality Prediction Model System (NAQPMS). The diurnal variations of the air pollutants and its mechanism were discussed. The results showed that NAQPMS can reproduce the temporal and spatial variations of the air pollution under complex terrain conditions. The concentrations of the pollutants presented a pattern with daytime maximum and night minimum, which can be explained by the daytime nearsurface temperature inversion layer.