A comprehensive mechanical characterization program on Green River Formation oil shale from the Mahogany zone near Anvil Points, Colorado, produced prediction equations for the three-dimensional non-linear stiffness coefficients based on the organic volume and stress level as independent variables. These coefficients are useful in the mine design, fragmentation modeling, investigations directed toward creating in-place permeability, and so on. The stiffness coefficients and compressive strengths perpendicular and parallel to bedding planes are obtained from tested samples. Each sample set consists of two square prisms taken from the same horizon but cut perpendicular and parallel to bedding planes. The twelve non-zero coefficients in the stiffness matrix for oil shale are expressed in terms of the elastic constants consisting of Young's moduli and Poisson's ratios obtained from uniaxial compression tests on these square prisms. Dependency on stress levels makes it difficult to determine the coefficients by acoustical methods.