The in situ Mg2Si nanoparticles (Mg2Sip)-reinforced AZ91D composites were fabricated by powder thixoforming that involved introduction of Si nanoparticles (Si-p) by ball milling followed by partial remelting and thixoforming. The results indicated that Si-p can be homogeneously dispersed on the surface of AZ91D powders, while they will be clustered when their amount exceeded 0.91 vol%. The thixoformed microstructure of the composites consisted of spheroidal primary alpha-Mg particles, intergranular secondary solidified structures (SSS) and in situ Mg2Sip that dispersed in the SSS. The tensile strength and elongation of Mg2Sip/AZ91D composites increased with the increased content of Mg2Sip, but the improved degree in the mechanical properties was reduced when its content exceeded 3vol%. The 4.5 vol.%Mg2Sip/AZ91D composite has the excellent mechanical properties, achieving an ultimate tensile strength (UTS) of 288 MPa, yield strength (YS) of 205 MPa and elongation (e(f)) of 6.9%, which were increased by 50% and 48.6% and decreased by 9.2%, respectively, compared to those of the AZ91D alloy. The strong Mg2Sip/Mg interfacial bonding and homogeneously distributed Mg2Sip were beneficial to activating the combined strengthening effects of thermal mismatch and Orowan mechanisms, improving the mechanical properties of the Mg2Sip/AZ91D composites. However, the Mg2Sip clustering degraded the strength of the composites with higher Mg2Sip content. [GRAPHICS] .