Abstract: Three biofuels, cornstalk, wheat straw and sawdust were combusted purely and co-combusted as given mixing ratio. The specific resistance (R) of ashes was measured by DR type dust specific resistance testing instrument and chemical compositions were detected by Atomic Absorption Spectrometry (AAS). The results show that the R values of ashes from pure biomass combustion are between 10⁸Ω·cm and 10¹0Ω·cm, which belongs to medium specific resistance. The R values of ashes from co-combustion are between 10⁸Ω·cm and 10¹2Ω·cm. Regarding ashes from burning cornstalk and coal, mixing ratio (R_t) brings little effect on R. As for ashes from combustion of wheat and coal, when the measuring temperatures are lower than 110℃, the more the R_t is, the lower the R is. The rule is inverse when the temperatures are higher than 110℃. Co-combustion alters the fuel characteristics of burning and fusing, changes the physical characteristics and chemical components as well as contents, resulting in the specific resistance differences between ashes from co-combustion and pure combustion.