A method to detect the intrinsic capacitance and thickness of graphene nanosheets (GNS) was developed. The ultrasonic-dispersed GNSs were separated by a kind of small-powder graphite (SG, 0.5–2 μm) with almost no capacitance to prevent the ultra-thin graphene layer from restacking face to face. In order to separate the GNSs as much as possible, we increased the mass ratio of SG in the GNS–SG composite gradually from 1 : 10 to 1 : 250. The electrochemical properties of different GNS–SG composites as electrode materials for an electric double-layer capacitor (EDLC) have been systematically investigated. It was found that, as the content of GNSs decreased from pure GNS to 1/100 of the GNS–SG composite, specific capacitance increased progressively from 171.8 F g⁻¹ to 417.1 F g⁻¹. As the ratio of GNSs in the GNS–SG electrodes was decreased further (1 : 200, 1 : 250 and 1 : 300), the specific capacitance held a stable value of ∼856.6 F g⁻¹, revealing the intrinsic capacitance of these GNSs. Based on the theoretical specific surface area of graphene and capacitance of a common graphite electrode, the layer number of GNSs here was calculated to be 2–11. This strategy can not only calculate the average layer number but also preliminarily judge the aggregation of GNSs from the capacitance data.