The effect and Condition of heat treatment on the tensile strength of welded joint was investigated in 300 grade 18% Ni-Co-Mo-Ti maraging steel sheets welded with electron beam. A good tensile strength of welded joint was obtained by following heat treatment cycle ; At 1100℃ the specimen was high temperature solution treated for 1 hour and then it was repeated solution treated at 900℃, 820℃ for 1hour respectively to recrystallize the coarsened γ grain. These heat treatment cycle was completed by an final aging heat treatment at 480℃ for 4 hour. Moreover, dissolution of dendrite, a significant decrease in seregation of Mo, Ti in weld metal were observed and also the coarsened γ grain formed at 1100℃, 1200℃ changed to fine grain due to the effect of recrystallization.