The dependence of the magnetic torque of single crystal GGG (Gd3Ga5O12) substrates on their geometry and some related physical parameters were studied. A method was proposed to deduce the substrate contribution in determining the uniaxial anisotropy constant Ku and magnetocrystalline one K1. Both Ku and K1 can be acquired from the magnetic torque curves by employing the two-variable regression and using computer. As example, the measured curves of Ku and K1 as a function of temperature were presented.