Inside the Clausius-Clapeyron regime, transformation stresses during supere-lastic tensile tests of polycrystalline shape memory alloys are linearly dependent on temperature, with coefficients being the slopes of the forward and reverse transformation lines. In this work, experiments are performed to investigate the anisotropy of the slopes of the forward and reverse transformation stress-temperature lines in a NiTi superelastic thin walled tube. The classical Clausius-Clapeyron relation is widely used to model these slopes, although, in a strict sense, this relation is defined at thermodynamic equilibrium. Experimental results disagree with the widely used classical Clausius-Clapeyron relation in two points: (i) that there should be no difference between slopes for forward and reverse transformations and (ii) that the products of the slopes by the transformation strains should not depend on orientation, since the remaining terms (mass density and entropy change) are not orientation dependent. A modified "Clausius-Clapeyron" relation is then proposed, better suited to model the anisotropy of the slopes of stress-temperature transformation lines of forward and reverse in superelastic NiTi. This modification is based on a unified thermodynamic theory of thermoelastic martensitic transformation in which irreversible energies are accounted as a sum of stored elastic energy and dissipated energy. The modified "Clausius-Clapeyron" relation is obtained by expressing that this irreversible energy is temperature dependent and that this temperature dependence is dependent on the orientation.