Abstract: By using observations of meteorological variables and PM2.5 concentration, together with the meso-scale numerical Weather Research and Forecasting (WRF) model, the weather conditions and boundary layer meteorological features were investigated during haze periods in Beijing in January 2013. The comparison with observations shows that WRF is able to reasonably reproduce the spatial and temporal distributions of the meteorological variables. Analysis of the heaviest haze periods, which occurred during 10-14 January and 27-31 January, shows that meteorological factors and high concentrations of atmospheric particulates are both responsible for the haze pollution. Small or calm wind and steady atmospheric stratification led to a decrease in atmospheric diffusivity and accumulation of pollutants. In addition, the southerly wind carried surrounding pollutants and water vapor to Beijing, which not only increased pollutant concentrations, but also favored aerosol hygroscopic growth, and extinction increase, which consequently led to haze formation and visibility decrease.