A quantitative method of number surface area of a graphene material includes the following steps. The graphene material is mixed with a solution to form a colloidal solution containing the graphene material. The colloidal solution is atomized to form a plurality of aerosols containing the graphene material. The size of the aerosols is screened. The screened aerosols are counted to obtain a number concentration of the screened aerosols. A surface of the screened aerosols is charged and a current amount on the surface-charged aerosols is measured. The number surface area of the graphene material is calculated based on the current amount and the number concentration.