The effect of replacing grain refining element Mn by Zr on fracture toughness of 2048 aluminum alloy was investigated. Fracture toughness changed by Zr replacement was analyzed as follows. 1. Quantity of large inclusions gave a primary effect on fracture toughness when grain size and material strength level were kept constant. 2. Dispersoids of Mn compounds were rod shape and 1.0㎛ in length, while those of Zr compounds, Al₃Zr, were spherical and 0.1㎛ in diameter. Dispersoids of Zr compounds, Al₃Zr, were better than those of Mn compounds for improving fracture toughness. 3. Experimentally proved the effects of dispersoids change on fracture toughness of aluminum alloy 2048 when other metallurgical factors-grain size, inclusion, strength level-were kept constant. Zr bearing modified 2048 aluminum alloy showed 5 % improvement in yield strength and 50 % in toughness compared with those of Mn bearing conventional 2048 aluminum alloy.