A aluminum base alloy containing boron and manufacturing method thereof, said alloy exhibiting good mechanical properties (such as high-temperature strength and creep strength) over a long period of time and also having a neutron absorbing capacity owing to boron present therein in the form of a compound without segregation. The alloy contains 0.5 10 mass % of boron with an isotopic element satisfying a relation of .sup.10B/(.sup.10B +.sup.11B).ltoreq.30%. said boron being present in the form of a boron compound which is 300 .mu.m or below in size. The alloy is obtained by melting at a temperature in excess of 950.degree. C. and cast at a temperature in the range of 800.degree. C. to 950.degree. C., in such a way that the molten metal is kept for 60 180.degree. seconds until it cools from 950.degree. C. to the casting temperature.