Introduction: Gypsum products are widely used in clinical dentistry for construction of dental models (casts) and dies. The ideal requirement of model and die materials include a high compressive strength and resistance to abrasion, in addition to high dimensional stability. Dental plaster, stone, and improved stone are the three types of gypsum products used in dentistry differ in their physical and mechanical characteristics. Although dental plaster is considered as the least expensive of the three gypsum products, but their low strength, microhardness and dimensional stability limit their use as mater cast and die materials. Aim: The aim of this study was to evaluate the effect of addition of 15 weight% aluminum oxide fillers on compressive strength, microhardness and setting expansion of the dental plaster. Materials and Methods: A total of 60 specimens; 30 specimens per each group were prepared. The control group was made by mixing dental plaster with water. The other group was made by addition of 15 wt.% aluminum oxide fillers to the conventional dental plaster prior to mixing with water. Water-powder was done according to the manufacturer?s recommendations among all groups. After 24 hours compressive strength, surface microhardness and setting expansion were evaluated by Universal testing machine; Vickers microhardness testing and extensometer respectively. Results: Dental plaster reinforced by 15 wt.% aluminum oxide fillers demonstrated higher mean compressive strength and microhardness values, while lower mean setting expansion values compared to that of conventional dental plaster (p < 0.05). Conclusion: Dental plaster reinforced by 15 wt.% aluminum oxide fillers could be used alternatively to type IV: high strength dental stone (improved stone) to be used as master cast and die materials.