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 $1100^{circ}C$ the specimen was high temperature solution treated for 1 hour and then it was repeated solution treated at $900^{circ}C$, $820^{circ}C$ for 1 hour respectively to recrystallize the coarsened ${gamma}$ grain. These heat treatment cycle was completed by an final aging heat treatment at $480^{circ}C$ for 4 hour. Moreover, dissolution of dendrite, a significant decrease in seregation of Mo, Ti in weld metal were observed and also the coarsened ${gamma}$ grain formed at $1100^{circ}C$, $1200^{circ}C$ changed to fine grain due to the effect of recrystallization.