Friction welding was conducted on the dissimilar alloys, Nimonic 80A and a martensitic heat resisting steel. Effects of specimen diameter and post heat-treatment on the microstructure and mechanical properties of weld joints were investigated. Metallographic and microhardness tests indicate that the weld region is composed of interaction layer, dynamically recrystallized zone, and heat affected zone. Interaction layer was composed of γ-(Fe, Ni) solid solution formed by mechanical mixing and interdiffusion. Hardness of the interaction layer reaches HV 700 due possibly to fine microstructure and high dislocation density. Post heat-treatment reduces hardness through decomposion of γ(Fe, Ni). The post heat-treatment improves tensile properties through modifying microstructures of the HAZ where flow pattern is formed in as-welded condition.