This paper focused on the effect of in-process cooling conditions (coolants and flow rate) on the temperature history, the tool applied forces, wear resistance, mechanical, and microstructural properties of friction stir processing (FSP) of Al–Si aluminum alloy. FSP was carried out using different compressed air and water coolants with different flow rates. The FSP tool force was measured experimentally using an especially designed load measuring system. Optical microscopy was used to probe the microstructures of the FSPed samples. The results showed that the Si particles size significantly decreases with increasing in-process cooling rate. Mechanical properties of each FSPed sample were also determined using hardness tests. Finally wear tests were conducted using a pin-on-disk tribometer.