To enhance the mechanical property and the bioactivity of composites, nano-hydroxyapatite (n-HA) modified monomer casting nylon-6 (n-HA/N) composites were prepared by in situ polymerization. During the synthesis of n-HA/N composite, the n-HA and caprolactam were mixed, melt and placed in the field of ultrasonic radiation. The differences between composite with ultrasonic and without ultrasonic were investigated. The tensile strength and the viscosity average molecular weight of the nylon matrix were measured. The results show that the molecular weight of the nylon matrix decreased firstly and it had the lowest value when the content of nano-hydroxyapatite was 1.6 wt.%. After that the molecular weight increased and then it began to decrease when it reached the highest value. But the tensile strength of the n-HA/N composite were improved. The ultrasonic dispersion made the n-HA more evenly dispersed in the nylon and increased the mechanical properties of the n-HA/N composites significantly. The bioactivity and moisture absorption of n-HA/N composites in simulated body fluid (SBF) were examined and compared to pure nylon. What's more, Fourier transform infrared spectrometer was used to characterize the structure of the materials formed on the surface of the composite. The results showed that moisture absorption of the n-HA/N composites was lower than that of the pure nylon. After composites impregnated 16 days in SBF, a layer of carbon hydroxyapatite (CHA) with weak crystalline was formed on the surface of sample. This phenomenon showed that the n-HA/N composites have good bioactivity.