In this study, precipitation behavior of Mg-Y-Nd cast alloy during friction stir processing (FSP), and the effect of subsequent artificial aging on mechanical properties and fracture behavior of the FSP alloy were investigated. It is found that the coarse alpha-Mg grains and large second phases are greatly refined after FSP. Moreover, due to the heat input during processing and the natural cooling, beta' and beta(1) precipitates are also observed in the FSP alloy. The FSP specimens were subjected to subsequent artificial aging treatment, and the peak hardness is obtained at 150 degrees C for 54 h and 180 degrees C for 30 h. Strengths of the peak-aged specimens are further increased, which is attributed to the large quantity of beta" and b1 precipitates, respectively. Meanwhile, elongations of the peak-aged specimens are both decreased. Due to the comprehensive effects of banded structures and fine grains, failure mechanisms of FSP and peak-aged specimens are all mixed ductile-brittle fracture mode. However, compared to the FSP specimens, different fracture paths are exhibited in peak-aged specimens.