Two C-Mn-Si steels with and without Nb micro-alloying are selected in the present study. Both these high carbon steels have been thermo-mechanically processed using Gleeble 3800 simulator. Optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscope (AFM) were utilised for microstructural characterisation. The austenitising temperature was varied from 1150 °C to 1200 °C followed by hot compression and subjected to free cooling to evaluate the influence of austenite grain size on transformed microstructure and mechanical properties. It is evident that higher austenite grain size is obtained for both types of steels subjected to higher austenitising temperature. The refinement of pearlite interlamellar spacing (<0.20 μm) with degenerated morphology as well as finer prior austenite grain size (<40 μm) is found to be more prominent for Nb micro-alloyed sample. The average hardness values of both the steels are higher for the specimens, treated at a higher austenitising temperature which is attributed to finer interlamellar spacing. Finally, the correlation between the evolving microstructure and resulting mechanical properties (hardness and yield strength) has been made.