Ultrafine AlN particles with a hexagonal prism shape were formed at high production ratios exceeding 90% by the reaction between nitrogen plasma and molten Al50Y50 alloy in the mixing N2+4%H2 gas of 40 kPa. The diameter and the length of the AlN particles are about 50 and 200 nm, respectively. The existence of YN particles with a size of about 5 nm is often observed in the AlN particles. The high production ratio and the unique morphology for the AlN particles are significantly different from the low production ratio of about 30% and the hexagonal plate shape for the AlN particles obtained from pure Al ingots by the same fabrication technique. The increase in the production ratio by the use of the Al-Y alloy is presumably due to the increase in the dissolution amount of N2 in the liquid resulting from the dissolution of the Y atom with an atomic size larger than Al and a large negative heat of mixing against N2. Furthermore, the achievement of the unique particle morphology is presumed to reflect the change in the preferential growth direction to the  direction resulting from the preferential occupation of Y at the sites with a loose atomic configuration along the direction which is perpendicular to the basal (0001) plane with a more dense atomic configuration.