The strength level of welded joint in room temperature and elevated temperature up to $600^{circ}C$ was investigated in 250 and 300 grade 18% Ni maraging steel sheet welded with electron beam. The results obtained in this study are as follows; 1. Optimum welding heat input was 600J/cm in 1.0mm thickness and the room temperature tensile strength, joint efficiency of welded joint treated with optimum aging condition were found to be about 166kg/$mm^2$, 95% in 250 grade, 189kg/$mm^2$, 92% in 300 grade maraging steel sheet, respectively. 2. Tensile strength of welded joint in room temperature increased slightly by aging after repeated solution heat treatment, but the fracture mode showed a shear. 3. Joint efficiency at a temperature between $540^{circ}C$and $600^{circ}C$ found to be about 72% to 55%, but the joint efficiency exceeded about 90% below $300^{circ}C$. 4. The fracture occurred in most weld metal, and the fracture surface showed a shallow dimple.