Information about distributions of temperature and gas species within the fuel bed is of great importance in studying the formation of pollutants in an industrial-scale biomass boiler. The main objective of this study was to investigate the gas composition and temperature in some available sections of the fuel bed of a 4 MW reciprocating grate boiler. The results showed that the temperature profiles of the grate bars and the fuel bed suggest significant temperature gradients versus the height of the fuel bed. The averaged NO, CO, CO2, and O2 concentrations measured in the fuel bed were about 80 ppm, 21, 11 and 1 vol. %, respectively.