Magnesium alloy AZ91 contains 0.1 to 0.3mass% manganese as an alloying element. Such manganese always reacts with aluminum and produces various compounds. Actually, numerous Mn-bearing particles are visible in polished surfaces under an optical microscope. Mn-bearing particles are also expected to be present in molten alloy. Some Mn-Al compounds are thus believed to be closely related to the superheating effect. However, it is not clear specifically what compounds exist in molten alloys and contribute to grain refinement, of superheating. In order to study this problem, we utilized a diffusionless process based on rapid cooling. AZ91 alloy was melted in a stainless steel tube and then injected onto a copper block rotating at high-speed, at temperatures of 973, 1023, 1073, 1123, 1173 and 1223K to obtain cast ribbon. Cast ribbons were analyzed by X-ray diffractometer, electron micro-analyzer and transmission electron microscope. These analyses show that the cast ribbons consist of single phase and that the structures are quite homogeneous, i.e., diffusionless solidification occurs due to rapid cooling. Cross-shaped particles including Mn, Al, Fe were often observed only when the melt was rapidly cooled and then injected at 973K.