This paper describes tensile properties of a peak-aged Mg-4Y-3RE alloy at room temperature to 823 K with 10(-5)-10(-1) s(-1). The Mg alloy exhibited high strength (> 250 Mpa) at room temperature to 473 K. However, the strength rapidly decreased at 573 K. It is suggested that a large decrease in strength at 573 K is attributed to grain boundary sliding. Also, elongation increased rapidly at 723 - 823 K. This is likely to arise from the relatively high strain rate sensitivity of about 0.3 due to the glide-controlled dislocation creep.