Abstract. In order to study the plastic flow during linear friction welding (LFW), copper particles were added as tracers on the surface of lead block. The flowing state of interior metal in LFW was simulated by the home-made simulator. Results show that the upset speed promotes the plastic flow of metal and the oscillation frequency restrains it. The upset speed influences the plastic flow of metal more significantly than the oscillation frequency. Only when the pressure reaches 800N and the increasing rate reaches 100N/s, can obvious plastic flow of lead happen in the direction perpendicular to the compression. But the plastic flow of lead in the compression direction is visible even if the pressure and its increasing rate are small.