The powder phases and structures of nano-magnesium powder prepared by high-energy ball milling and gas condensation method, i.e. resistance-heating evaporation, are characterized by means of XRD and TEM. Nano-Mg/PP composites are prepared by melt milling polypropylene, in which process the surface of nano-magnesium powder is modified first and then put into the melted PP. The results show that nano-magnesium powder prepared by the milling method improves the tensile strength of the polypropylene matrix and the powder prepared by gas condensation improves the toughness of the polypropylene matrix notably and maintains or improves the strength to some extent at the same time. The SEM observation of the cross-section of the composites shows that magnesium powder disperses in the matrix at nanometer size. The toughening mechanisms of nano-magnesium powder prepared by the condensation method can be compared to the cool tensile mechanism of rigid toughening.