The development of new advanced high strength steels (AHSSs) with improved mechanical properties has awakened a great interest of the scientific and the industrial community over the last years. Particularly, great attention is currently being paid to the development of microstructures through smartly designed Quenching and Partitioning (Q&P). However, it is not clear yet which is the role of specific microstructural parameters, such as the prior austenite grain size (PAGS), on the complex microstructural evolution of Q&P steels. For this reason, the main goal of this work is to investigate how the PAGS modifies the size distribution and morphology of final Q&P microstructures and, in turn, how these changes are reflected in the final mechanical properties. For this purpose, a low carbon steel containing Mn and Si has been fully austenitized at different annealing temperatures for different times to generate different PAGSs. Subsequently, the material has been quenched to a selected quenching temperature and specific partitioning conditions. The microstructural evolution has been characterized by dilatometry, optical microscopy and scanning electron microscopy and the volume fraction of retained austenite has been determined by X-Ray Diffraction. The effect on the mechanical properties has been evaluated by micro-hardness tests. The results of this investigation provide a better understanding of the mechanisms underlying the microstructural changes and, therefore, will enable the optimization of the Q&P parameters leading to improved mechanical properties.