Biomass combustion is currently widely used for residential heating within the European Union instead of coal or wood combustion. Solid biofuels are classified as highly reactive low-calorific fuels. One of the most efficient and environmentally friendly ways to utilize them is under fluidized bed conditions (FBC). However, the harmful by-products during biomass thermochemical decomposition still must be considered, such as particulate matter (PM). The atmospheric air quality monitoring is currently regulating PM10 and PM2.5, but particles of submicron size (PM<1) are being observed for having the major fraction in the flue gases of coal, wood or different types of biomass combustion. The present work aimed at studying thermal behavior and PM size distribution during oxidation of agricultural residue in FBC. A contemporaneous thermal analysis (TG-DTA/DSC) studied the effect of weight loss, thermal stability and energy at constant heating rate of 10 ºC/min and dynamic mode (20 ÷ 1100 ºC). Three general stages were distinguished: (1) dehydration, (2) devolatilization and volatiles ignition and (3) char combustion. Two types of biomass residues, namely peach and apricot stones were utilized in a formation rate unit at atmospheric pressure and temperatures: 700 ÷ 900 ºC. The primary gaseous products of carbon and nitrogen conversion and their reaction rates were experimentally measured simultaneously with the PM number density in the exhaust, short after the reactor’s outlet. The obtained unimodal PM size distribution confirmed that the normalized mass number density of the PM<1 is three orders of magnitude higher than those measured for PM2.5 and PM10.