The microstructure evolution during thermo-mechanical processing of high-Nb HSLA steel has been investigated with laboratory investigations. Using the Gleeble 2000 thermomechanical simulator, constitutive behavior, recrystallization, and precipitation were quantified with single- and double-hit tests as well as isothermal-deformation-quenching tests. The critical strain (epsilon(c)) for the onset of dynamic recrystallization in high-Nb steel is derived and the result shows that the critical strain/peak strain ratio is as low as approximately 0.35 and tends to a constant when the effective Nb content (Nb-eff = Nb - Mn/120 + Si/94) ranges from 0.07 to 0.10. The interaction between the recrystallization and precipitation was considered to determine non-recrystallization temperature (T (nr)) under various conditions and further the dependence of the T (nr) on initial austenite grain size, strain, and strain rate on was formulated.