[en] The effect of microstructural parameters on the plane strain fracture toughness of scandium doped Al-6Mg alloys has been studied. Al-6Mg alloys with trace addition of scandium are subjected to annealing at different temperatures. Plane strain fracture toughness property (K_IC) of the alloys under different thermal conditions is measured. It is found that fracture of coarse insolubles adversely affect the fracture toughness of binary Al-6Mg alloy. Scandium treated alloys have shown improvement in fracture toughness due to the reduction in size and amount of second phase particles. The isochronal age hardening behaviour of low scandium alloy is found similar to the variation of fracture toughness with ageing temperature. However, high scandium alloy records a minimum in fracture toughness value at the peak age hardening temperature due to extensive formation of planar slip bands