Abstract: Friction plug welding is a novel solid state joining and repairing process. Based on the introduction of basic principle of friction plug welding process, preliminary studies on microstructure and hardness distribution of friction plug welded joint were carried out. It is shown that five zones could be easily identified in LY12 joint, i. e.the base metal, plastic zone Ⅰ, the transition zone be-tween base metal and plastic zone Ⅰ, plastic zone Ⅱ, the transition zone between plastic zone Ⅰ and plastic zone Ⅱ. Drastic material de-formation and metal flow occurr in plastic zone Ⅰ and Ⅱ, but these two zones have different flow patterns. Hardness of the joint decreas-es from the base metal and reaches a minimum in the plastic zone Ⅰ, and then gradually increases to a stable level in plastic zone Ⅱ which equal to the base metal.The other side of the joint has the same trend of hardness distribution.