Abstract: Al₂O₃/Mo₅Si₃ nanocomposite powders was synthesized by mechanochemical reduction method using MoO₃, Mo, Si and Al powders as the raw materials. The solid-state reaction process were studied. The results indicate that a mechanically induced self-propagation reaction is triggered during mechanical alloying of MoO₃-Mo-Al-Si powders. The main phase of composite powders are Mo₅Si₃、 Mo₃Si and Al₂O₃ after milling for 30 h. The content of Mo₃Si and the burning loss of Si increase in the composite powder with the increase of Al₂O₃ content. The particle size of the composite powder is small which between 1 μm and 3 μm. The grain size of Mo₅Si₃、 Mo₃Si and Al₂O₃ are 31~79.3 nm with nano-crystalline structure of the Al₂O₃/Mo₅Si₃ composite powders with mass fraction of 20%~40% Al₂O₃ after milling for 30 h. Al₂O₃/Mo₅Si₃ composite powder is very stable during annealing at 1000 ℃ for 1 h and no transformation takes place.