The Clausius-Clapeyron equation is modified in such a way that it can be used when the martensitic fraction is not a constant, and a model describing the relationship between recovery stress and temperature is developed based on the modified Clausius-Clapeyron equation. All the parameters in the model can be determined by experiments. The relationship between recovery stress and temperature is calculated by selecting the most often used property parameters of TiNi shape memory alloys. The calculation results agree well with the experimental data even when the prestrain is relatively large.