Abstract: Interfacial diffusion behavior during melting joining of thermoplastic polymer was studied using molecular dynamic simulation method.Two-layer cell interface comprising model for polymethyl-methacrylate(PMMA) was constructed.Under different temperatures and pressures,interfacial molecular chains motion was simulated using constant temperature and pressure(i.e.NPT) ensemble,diffusion coefficient of total atom of PMMA interface system,interfacial diffusion depth and binding energy were calculated,and influencing rule of factors such as temperature and pressure to interface diffusion behavior was analyzed.Simulated results show that by heating or pressing atoms diffusion speed was improved,interfacial diffusion depth was increased by higher pressure,more interfacial binding energy was gained by higher pressure.The simulation results agreed well with the macro hot-embossing experiments,and revealed the molecular diffusion behavior between PMMA interfaces during melting joining from molecular size.