The paper presents experimental investigation on temporal release of potassium (K) from single biomass pellet combustion by Flame Emission Spectroscopy (FES). Through the analysis of spontaneous emission spectra and a calibration procedure, the concentration of gas phase potassium, temperature and thermal radiation during the whole process of biomass pellets combustion in a Hencken burner were measured simultaneously. Camphorwood and rice husk with volatile matter 87.93%, 69.12%, respectively, and K2O content from the ash analysis 7.99%, 1.8%, respectively, were used in the experiment. The effects of biomass composition and moisture content in the pellet on the temporal release of K were analyzed. Experimental results showed the three stages (devolatilization, char, and ash stage) of biomass pellets combustion can be distinguished by the changing points of K concentration, temperature and thermal radiation curves obtained by the FES. The volatile content in biomass pellet has effect on the release behavior of K. The change of moisture in biomass pellet may affect significantly the ratio of K release from biomass pellet with low volatile. The experimental investigation proves FES is feasible for on-line monitoring the K release and combustion process of biomass pellet.