Abstract: Wood fiber-high density polyethylene composites (WPCs) with nano silicon dioxide (nano-SiO2, 2%, 4%, 6%) and ammonium polyphosphate (APP, 8%, 10%, 12%) were prepared to investigate the synergistic effects on interfacial and mechanical properties in WPCs using Fourier infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and mechanical measurements. Results showed that: 1) SEM observation on the fracture surface of the composites indicated that APP and nano-SiO2 exhibited uniform diffusion in WPCs with 2% -6% nano-SiO2 and 8% -10% APP. And the presence of Si—O— C in WPCs due to interactions between nano-SiO2 and wood fiber could improve the interfacial characteristics. However, the agglomeration was observed in WPCs with 12% APP. 2 ) With incorporation of 2% -6% nano-SiO2 and 8% -10% APP, WPCs showed the higher tensile and flexural strength, similar tensile elongation at break, and lower impact strength. The results of two-way ANOVA analysis showed that the differences in the tensile strength, tensile elongation at break, and flexural strength were not significant among nano SiO2 addition, APP addition and interactions between nano SiO2 and APP at the level of 0.05, while impact strength showed the significant differences among them at the level of 0.05.