Poly(L-lactic acid)/poly(L-lactic acid)-grafted SiO2 nanocomposites were prepared by in situ melt polycondensation, in which "free" poly(L-lactic acid) and poly(L-lactic acid)-grafted SiO2 nanoparticles were formed simultaneously. The maximum values of grafting ratio and grafting efficiency of poly(L-lactic acid) were up to 37.67% and 26.60%, respectively. In the polycondensation system, SiO2 content was a critical parameter of getting nanocomposites with uniformly dispersed SiO2 nanoparticles. At lower SiO2 content, Mn of grafted poly(L-lactic acid) was close to that of "free" poly(L-lactic acid), and poly (L-lactic acid)-grafted SiO2 nanoparticles could be well dispersed in poly(L-lactic acid) matrix. While at higher SiO2 content, Mn of "free" poly(L-lactic acid) and grafted poly(L-lactic acid) decreased seriously, especially GPC curves of "free" poly(L-lactic acid) exhibited two peaks due to the aggregation of SiO2 nanoparticles during the polycondensation process. The grafting ratio and SiO2 content exhibited a clear effect on the thermo-oxidative degradation of nanocomposites. The existence of poly(L-lactic acid)-grafted SiO2 nanoparticles dramatically improved the thermo-oxidative stability of poly(L-lactic acid). Compared with that of pure poly(L-lactic acid), T-g, T-c, and T-m of nanocomposites varied slightly.