The strength of a final product of steel is affected by its final austenite grain size. Almost applied models for grain growth based on Beck and Sellars equation, which has the isothermal condition assumption; whilst most of the materials processing take place under non-isothermal condition. The purpose of this research is to find the effect of Nb and cooling rate to the austenite grain growth model of HSLA-Nb steel to predict the Austenite grain size after hot rolling process in non-isothermal condition. The two composition of 0.019 and 0.056% Nb of HSLA-Nb was hot-rolled about 0.3-0.4 deformation at a temperature of 900-1100°C, followed by cooling rate of 7-11° C/s, in a time period of 25-40 second, and quenched by using water jetspray. Austenite final grain size was measured. The work shows that the higher Nb content of steel with higher cooling rate will reduce the final austenite grain size. It was also found that the new non-isothermal austenite grain growth after hot-rolled illustrated as a function of cooling rate, which obtained by modifying the previous established model by a cooling rate parameter 1/Cr m, and it was more precision compared to previous grain growth equation.