Flue gas from an ultra-low emission power plant has higher relative humidity and lower particulate matter concentrations than flue gas from typical power plants. Humidity can influence particulate matter concentration measurements. This study used an Electrical Low Pressure Impactor (ELPI) system to study particle size distribution under different humidity conditions. The results show that the total concentration of particulate matter increased with increased humidity. Heterogeneous condensation resulted in an increase in particulate matter larger than 0.02 mu m and a decrease in particulate matter smaller than 0.02 mu m. Condensation of water vapor caused a significant increase in the amount of 0.1 mu m sized particles. Collision-coalescence of particulate matter resulted in a significant increase in the amount of particles greater than 2 mu m in size. When relative humidity is below 33.4%, a Nafion drying tube can be used to reduce the humidity level and obtain approximately accurate particle sampling results. For relative humidity conditions exceeding 33.4%, a diluter and a Nafion drying tube must be used simultaneously for dehumidification to ensure accurate results. Samples taken at an ultra-low emission power plant in China show good correlation with laboratory scale samples under high humidity conditions (73.7%). The PM0.2, PM1, PM2.5, and PM10 particle sampling results without dehumidification are 2500%, 1500%, 1000%, and 1400% higher, respectively, than the results obtained using both the diluter and Nafion drying tube.