Carbonization experiments of grapeseed and chestnut shell samples having the average particle size of 0.657 mm and 0.377 mm, respectively, were performed to determine the effect of temperature, sweep gas flow rate and heating rate on the biochar yield. A statistical design technique was applied by the use of a two-level factorial design matrix to interpret experimental results. Carbonization conditions were selected according to a two-level factorial design matrix considering the following variables: temperature (723 K and 823 K), nitrogen gas flow rate (0 and 1000 cm3/min) and heating rate (5 and 20 K/min). Empirical relations between the biochar yield and the carbonization conditions were developed. To comment on the effect of parameters between the superior and inferior levels and to prove the accuracy of design equations from statistical design technique, biochar yields obtained at different conditions were also presented in the graphical way. It was found that temperature has the strongest effect on the biochar yields in comparison with nitrogen gas flow rate and heating rate. Biochar yields of grapeseed and chestnut shell were decreased with the increasing temperature, heating rate and sweep gas flow rate. A comparison between the fuel properties of waste materials and biochar products was also done. © 2008 Elsevier B.V. All rights reserved.