Abstract: To investigate the slow pyrolysis kinetic process of the nutshell biomass, thermogravimetric analysis experiment at inert atmosphere of two species of nutshell biomass (peanut shell and sunflower hull) was performed. A competitive pyrolysis reaction model was used to explain the pyrolysis of the nutshell biomass. A new empirical kinetic mechanism function was put forward based on the existing kinetic mechanism functions. Kinetic analysis for the experimental data was carried out using pattern search method, a modern optimization technique. The results show that the new kinetic mechanism function can very effectively describe the slow pyrolysis kinetic process of nutshell biomass.