In this research, AZ31 alloy and AZ31 alloy reinforced with graphene oxide particles (i.e., AZ31-GO nanocomposite) were subjected to friction stir processing (FSP) and their microstructure and mechanical properties were compared. The FSP process was carried out under a constant advance speed of 85 mm/min, rotation speeds of 1180 and 1500 rpm, and shoulder diameters of 12 and 16 mm. After applying the FSP process, the grain size in the AZ31 and AZ31-GO nanocomposite samples reached from 80 micrometers to about 3.5 and 3.7 micron, respectively. Hardness in FSP samples increased by about 36% and in composite samples by about 28%, which increase in hardness can be attributed to fine graining through dynamic recrystallization, pinning of boundaries by secondary phase particles and reinforcement particles and prevention of dislocations climbing by reinforcing particles. Microstructural observations showed that increasing the rotation speed and shoulder diameter causes a homogeneous and uniform distribution of graphene oxide in the microstructure. The tensile test was performed with a strain rate of 0.05s-1. Elongation and UTS in FSP and nanocomposite sample with 12 mm shoulder diameter reached from 38.082 and 212.8686 MPa to 37.5 %, 246.9484 MPa, and 31.74 %, 257.0776 MPa, respectively.