The particles and inorganic gaseous compounds from a 5 MW moving grate biomass boiler were characterized at two sampling positions at 1100 °C and 800 °C respectivly. Samples were extracted with high temperature sampling method with primary dilution ratio of 20. For both sampling positions a biomodal mass size distributions were observed, with ultrafine mode at around 70-100 nm and fine mode at 10-30 nm. The increasing concentration of ultrafine particle over time indicates that the alkali deposits in the inner wall of dilution probe re-evaporates and condenses on the ultrafine particles. The elemental analysis and ion ratio of (K+Na)/(Cl+2S) and 4S/O indicates that at 1100°C the main species composted of particulate matter are not only alkali chlorides and alkali sulfates.