Abstract: Hexagonal boron nitride (h-BN) was exfoliated by ball-milling method with assistance of melamine. The thicknesses, morphologies and structures of as-prepared melamine-functionalized boron nitride nanoflakes (f-BNNS) were characterized. By investigating the f-BNNS-filled polyvinyl alcohol (PVA) composites, the effects of exfoliation of h-BN and f-BNNS on mechanical properties of composites were studied. The mechanism of interface interaction between f-BNNS and PVA matrix was also analyzed. The results show that most of the f-BNNS exhibit regular rounded flake-liked shape with diameters and thicknesses concentrated on the ranges of 1-3 nm and 150-400 nm, respectively. Comparing with h-BN-filled h-BN/PVA composites, f-BNNS can be well dispersed in PVA matrix. In mean time, the yield strength and elastic modulus of f-BNNS/PVA composites are both improved. When the mass fractions of f-BNNS achieves at 5wt%, the yield strength and elastic modulus f-BNNS/PVA composite films are 175.2 MPa and 4.52 GPa, which are 45.3% and 53.7% higher than that of neat PVA, respectively. From the molecular structures of PVA and f-BNNS and the SEM images of fracture surfaces of f-BNNS/PVA composites, the hydrogen-bonding interaction between the amino groups of f-BNNS and the hydroxyl groups of PVA chains effectively enhances the interfacial interaction between f-BNNS and PVA matrix.