Fulvic acid-thiourea amide derivates (FA-T) was synthesized via amidation with FA and thiourea, and its optimum reaction conditions were 7 h of reaction time, 1 wt% of sodium methylate, 130 degrees C of reaction temperature, and 2 g of thiourea. Then, FA-T grafted polystyrene (FA-T-PS) was synthesized by activators generated by electron transfer for atom transfer radical polymerization. Fourier transform infrared spectroscopy, static contact angle analysis, and X-ray photoelectron spectroscopy confirmed that the synthesis of FA-T-PS. Then, poly(lactic acid)/FA-T-PS(PLA/FA-T-PS) composites were prepared by the melt blending with FA-T-PS as fillers. Mechanical test demonstrated that FA-T-PS increased the flexibility and ductility of PLA composites. Dynamic mechanical analysis revealed that FA-T-PS reduced friction and loss between PLA chain and filler, and further reformed had higher interfacial compatibility with PLA. Differential scanning calorimetric results and polarized optical microscopy analysis displayed that FA-T-PS had strong heterogeneous nucleation effect, which effectively enhanced the crystallization rate and the crystallinity of PLA. Friedman thermal decomposition kinetics presented that E-a of PLA/FA-T-PS (0.3 wt%) was increased by 52.94% compared with PLA, which demonstrated that FA-T-PS significantly enhanced the thermal stability of PLA. Therefore, FA-T-PS effectively improved the comprehensive performance of PLA. POLYM. ENG. SCI., 59:1787-1798, 2019. (c) 2019 Society of Plastics Engineers