Abstract: The microstructure of an in-situ reactive spray formed TiCp/7075 (TiCp = 2.91% in volume fraction) Al alloy was observed using SEM. The grain sizes were measured by Image Tool software and a mean linear intercept method and compared with that of spray formed 7075 Al alloys. The result showed that the grain size of the former alloy was decreased 50 % compared to the later. It is indicated that in-situ TiC particles play a significant role on refining the microstructure of spray formed 7075 Al alloys. The refinement mechanism was interpreted from nucleation attribute, nucleation velocity and crystallographic analysis in nucleation process.