Abstract The mechanism of reinforcing rubber with carbon black particles is investigated by comparing the strain energy density functions of pure and reinforced rubbers. Finite deformation biaxial extension experiments are used to measure the strain energy density. The functional form of the reinforced system, which has been proposed by Kawabata, is examined and its validity was confirmed over a temperature range of −20°C to 80°C. The functional form is: W(I1,I2)=CT(I1−3)+β(I1,I2)+γ(I1), where I1 and I2 are the invariants of the deformation tensor. Parameter C is a constant and T is the temperature. The reinforcement effect on W appears as the γ function which is not affected by temperature. The results are analyzed with respect to a network mechanical model consisting of molecule bundles connecting carbon-black aggregates. The function W is calculated and it is shown that the functional form of Wis very similar to that of the reinforced rubber.