Semi-solid formings of an AZ91 magnesium alloy containing various sizes and forms of α-phase particles are prepared by a newly developed method. The effects of these structures on the tensile properties and elastic-plastic fracture toughness (JQ) are investigated in as-cast and in heat-treated samples. The obtained results are as follows : (1) As-cast samples show small values of both tensile properties and fracture toughness regardless of grain size and these are due to the fact that cracks propagate mainly in brittle eutectic compounds. (2) Both tensile properties and fracture toughness of T4-and T6-treated samples increase as grain size decreases. This is assumed to result from the fact that (a) stress concentration at a grain boundary is lower and more twinning deformations occur in a grain as grain size decreases and (b) eutectic compounds are dissolved quickly into the matrix by the short solid solution treatment in semi-solid formings with small grain size.