Phase equilibria of AZ91D Mg alloys for nonflammable use, containing Ca and Y, were carried out by using FactSage<SUP>Ⓡ</SUP> and FTL database, which revealed that solution treatment could be performed at temperatures from 400 to 450℃. Solid solution treatment of AZ91D Mg alloy without Ca and Y was successfully conducted at 420℃ and supersaturated microstructure with all beta phase resolved into matrix was obtained. In the case of AZ91D Mg alloy with some Ca and Y, however, a little amount of intermetallic particles were observed after solid solution treatment. After solid solution treatment, each alloy was annealed at temperatures of 180 and 200℃ for time intervals from 1 min to 48 hrs and hardness of each condition was measured by micro-Vickers method. Peak aging conditions were deduced as at the temperature of 200℃ for 10hrs.