In the present research, the effects of different friction welding parameters including the welding pin’s traverse and rotational speeds and the number of weld passes as well as the effect of adding TiO2 nanoparticles on the mechanical properties and microstructures of friction stir welded copper sheets have been investigated experimentally. This study shows that for achieving more desirable welds, instead of increasing the number of weld passes, it is better to conduct the welding process at a lower temperature. It is also found that friction stir welding can increase the Vickers hardness of the stir zone by up to 71.4% (without TiO2 nanoparticles) and by 85.7% (with TiO2 nanoparticles). Also, the impact toughness of friction stir welds can be increased by up to 80% (without TiO2 nanoparticles) and by 94.5% (with TiO2 nanoparticles). However, the addition of nanoparticles has no effect on the tensile strength of resulting welds.