This article deals with biaxial low cycle fatigue (LCF) behavior of the material SA333 C-Mn steel in tension-torsion experiments. Two sets of experiments have been conducted with this material for tension-torsion loading with tubular specimens. The first set is for in-phase loading with strain ratio (shear strain/axial strain) 1.732 for different plastic strain amplitudes. The second set of experiments is for 90° out of phase for the same strain ratio for different plastic strain amplitudes. The experimental observations show extra hardening due to non-proportionality in 90°out of phase loading. The non proportionality index is calculated from Benallal [1] criterion. Finite Element simulations of the tension-torsion experiments have been carried out. The material model has been developed in the framework of von-Mises cyclic plasticity theory. Ohno-Wang nonlinear kinematic hardening rule, modified by Jiang-Sehitoglu [2] has been used. Cyclic hardening/softening is introduced through isotropic hardening rule. The cyclic hardening/softening is split into two components, namely the proportional part and the extra hardening due to non proportionality in loading. The material constants have been derived from the experimental results. The non-proportional hardening is calibrated by the Benallal non-proportionality index. Axial stress-strain loops and Shear stress-strain loops are simulated for both in-phase and 90°out-of-phase tension-torsion loadings. The simulated results are compared with the experimental results.