Nanocalcium carbonate (CaCO) was successfully adhered to the surface of bamboo fiber (BF) via both impregnation and blending modification. The BF-, BMBF (bamboo fiber treated by blending modification)- and IMBF (bamboo fiber treated by impregnation modification)-reinforced high-density polyethylene (HDPE) composites were all manufactured by means of extrusion molding. The flexural and impact properties of the composites (the addition of BF, BMBF and IMBF were all 30 wt%) were analyzed. CaCO with a loading of 15 wt% had an effect on the performance of the composites. The flexural strength (FS) of the BMBF and IMBF composites increased by 1.09 and 9.36%, respectively, while the differences of the impact strength were insignificant among these, compared to the BF/HDPE composites. The flexural properties of the IMBF/HDPE composites were investigated with different mass fractions of IMBF (5, 10, 15, 20, 30, 50, 60 and 70 wt%). The results showed that the FS of the IMBF/HDPE composites reached a maximum value (58.99 MPa) when the mass fraction of the IMBF was 30 wt% and increased by 50.95% compared to when the mass fraction was 5 wt%. These results were supported by ESEM and fractal dimension analysis in terms of proper distribution of nano-CaCO and interfacial adhesion between the IMBF and HDPE matrix. The results revealed that the fractal dimension D of IMBF/HDPE composite with a mass fraction of 30 wt% reached a maximum value (2.2036), which was similar to the FS results. There was a linear correlation between lg (FS) and fractal dimension D, indicating that the fractal dimension was practicable for the IMBF-reinforced HDPE composites. The fractal features could reflect the macro-mechanical properties, and the percentage error of the fitting function was within 10%. [ABSTRACT FROM AUTHOR]