The hydraulic fracturing is a major means of the coalbed methane（CBM） exploitation

and the effective channels of the methane seepage can be formed in the coal seams by this means. The stimulated reservoir volume（SRV） can not only help the coal seams form the seepage channels

but also increase the desorption-diffusion areas of the methane and improve the production of the single well by formed the complicated fractures. The brittleness index is one of the key parameters for evaluating the SRV implement

therefore

based on the stress-strain curves of the coal and shale

it is analyzed with the help of the strain-energy density. The higher the brittleness index of the shale is

the lower the strain-energy density will be. Based on the calculation method of the brittleness index of the shale

the calculation method of the coal brittleness index that combines with the strain-energy density and minimum energy principle is put forward

and the modified brittleness index of the coal is obtained

which can be used for the characterization of the coal brittle failure and evaluation of the fracture formation after fracturing. Finally

the analysis of the calculation results and data of 6 fracturing wells

indicates that the strong correlation between the coal brittleness index and stimulated volume

which means that the higher the brittleness index is

the larger the stimulated volume of unit fracturing fluid in the coal seams will be. This method can be applied to design the SRV of the coal seams and guide the exploitation of CBM.