Heat dissipating plate (4) made of copper-base alloy is proposed that exhibits high degree of flatness after joining in the step of assembling power semiconductor modules, IC packages, etc., that will not crack in the solder (3) joint if subjected to heat cycles during joining or in an environment of use, and that has high heat conductivity and cost effectiveness. The heat dissipating plate (4) uses a copper-base alloy having a 0.2% yield strength of at least 300 N/mm.sup.2 which is characterized in that the 0.2% yield strength after heating at 400.degree. C. for 10 minutes is at least 90% of the 0.2% yield strength before heating and that said copper-base alloy has a heat conductivity of at least 350 W/mK and contains at least one element of the group consisting of Fe, Co and Ni plus P in a total amount of 0.01 0.3%; the heat dissipating plate (4) is 10 200 mm long on each side, 0.1 5 mm thick and warped by 200 .mu.m or less in a curved shape with a radius of curvature of at least 100 mm; this heat dissipating plate (4) exhibits high degree of flatness after the assembling step and assures improved heat dissipating performance.