Magnesium has a close-packed hexagonal structure, but the ductile/brittle fracture behaviour in magnesium alloys is not well known. Temperature dependence on the impact strength (absorbed energy) has been examined by means of Charpy impact test. Three types of extruded materials were tested at temperature range from 93K to 523K. Specimens of 99.95% pure magnesium, Al3%Zn1%Mg(AZ31) and A16%Zn1%Mg(AZ61) alloys were annealed and tested in order to study the effect of microstructure on impact strength. For AZ31 and AZ61 alloys at each temperature of 293K and 423K, load-displacement (or time) curves were also obtained by means of an instrumented Charpy impact test. The fracture initiation energy and fracture propagation energy have been calculated from these data. Ductile-brittle transition temperature was estimated from the impact strength curve as a function of test temperature, and the values were around 393K for AZ31 and 443K for AZ61. The impact strength curves show same tendency and values in both pure magnesium and AZ61 alloys. Impact strength for AZ31 at 473K, however, showed two times larger compared with those of other two specimens. From the result of load-displacement curve, it seems that impact strength of AZ31 increases with increasing both the fracture initiation and the fracture propagation energies at higher test temperatures, however, for AZ61, only the fracture propagation energy contributes to the impact strength. Significant difference in impact strength was not observed between extruded and annealed specimens. In quenching condition, the impact strength increases up to 117%, and in ageing condition, decreases to 60% compared with the strength of extruded condition.