This study investigated the influence of particle size on anaerobic biohydrogen production from wheat stalk by mixed microflora. In addition, the kinetic model for the formation of main products was also mentioned. The results demonstrated that all the cumulative productions of hydrogen, acetate and buty-rate decreased as the particle size increasing from 1 to 10 mm at a constant TS value of 2%, 5% and 8%, respectively. However, this difference for aqueous products was not very obvious compared with hydrogen. A modified Gompertz equation was able to adequately describe the cumulative production of hydrogen, acetate and butyrate (R(2) higher than 0.989). The results also indicated that the formation of the main products were all associated with the degradation of cellulose and hemicellulose (R(2) higher than 0.855). (C) 2011 Elsevier Ltd. All rights reserved.