Abstract: The PET/boehmite (BM) composites were prepared using BM as a flame-retardant and reinforcement additive, and the effects of BM dosage on the mechanical, crystalline, thermal and flame-retardant properties of the composites were investigated. The experimental results indicated that the rigidity of the composites increased gradually with an increase of BM content, but the toughness tended to decrease gradually. The Tc and Tcc of the composites were found to increase and decrease gradually with an increase of BM content, respectively. However, the value of ΔT·(Tc-Tcc) shows a gradual increase. BM could act as a nucleation agent to promote the crystallization rate and the relative degree of crystallinity (Xc) of PET domain by heterogeneous nucleation. Nevertheless, the melting temperature of the composites presented a decrease with increasing the content of BM. BM could also generate good flame retardant effect on PET. The incorporation of BM resulted in an increase in char yield and a decrease in decomposition temperature for the composites. As a result, the thermal stability of the composites was improved during combustion. In summary, BM plays an important role in enhancement of mechanical, crystalline and flame-retardant properties for the PET/BM composites.