The dependence of the martensite start temperature on the austenite grain-size is modeled based upon aspects of martensite nucleation and validated by data reported in the literature. The results obtained support the view that grain boundaries provide an environment for the formation of martensite embryos in annealed austenite. The model can account for a two-stage dependence of martensite start temperature on the austenite grain-size, considering that grain boundaries provide a suitable environment for martensite nucleation, and grain-size also has a stabilizing effect by limiting the volume of individual martensite units.