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Mg nanoparticles were prepared by an inert gas condensation method using magnesium powder of 99.9% purity, and the effect of evaporating temperature and deposition location on the morphology and size of Mg nanoparticles were studied. Result shows that chain-like Mg nanoparticles with size of 10~50 nm are prepared in the front tip of the cold trap at the evaporation temperature of 600 oC. Some Mg nanowires are observed among the nanoparticles. Smaller size Mg nanoparticles are obtained at lower evaporation temperature in the front and back tips of the cold trap. According to collision and coagulation mechanism in the gas-solid phase transition theory, the formation process of Mg nanoparticles was explained and the corresponding model was established.