The phenomenological strain energy density function (Vn for the elastomeric networks of end-linked poly(dimethylsiloxane) (PDMS) has been investigated as a function of the first and second invariants I-1 and I-2 of the Green's deformation tensor on the basis of the quasi-equilibrium stress-strain relationships of general biaxial deformations varying independently each of two principal strains. The 1, dependence of partial derivativeW/partial derivativeI(j) (i,j = 1,2) was obtained from the biaxial stress-strain data using the Rivlin-Saunders method. In the 3-dimensional plots of partial derivativeW/partial derivativeI(i), (i = 1,2) against both the (I-1 - 3)- and (I-2 - 3)-axes, the data points of each derivative at large deformations appear to fall on a plane inclining against the (I-1,I-2) plane, which suggests that both the derivatives linearly depend on each of I-1 and I-2, The formula of W is reasonably deduced from such linear dependence of partial derivativeW/partial derivativeI, on I-j (i,j = 1,2) as W = C-100(I-1- 3) - C-01(I-2 - 3) + C-11(I-1- 3)(I-2 - 3) + C-20(I-1 -3)(2) C-02(I-2 - 3)(2). Each of the numerical coefficients C, is assigned to each of the intercepts at I-1 = I-2 = 3 and the gradients of the two fitted planes in the (I-1, I-2, partial derivativeW/partial derivativeI(i)) plots. The estimated W satisfactorily reproduces not only the original biaxial stress-strain data but also the data of uniaxial, equibiaxial elongation, and uniaxial compression none of which were used for the original estimation of W. It is also demonstrated that the familiar Mooney-Rivlin type of W composed of only two linear terms of each of I-1 and I-2 does not even qualitatively reproduce the biaxial stress-strain data.