Abstract: A tool box assembly contains plural tool boxes with a same size or different sizes, and each tool box has a first casing and a second casing. The first casing includes at least one groove defined on a top thereof, and the first casing also includes at least one engaging portion arranged on a peripheral side of the at least one groove. The second casing covers with the first casing and includes at least one protrusion mounted on a bottom thereof and corresponding to the at least one groove of the first casing. In addition, each of the at least one protrusion has a joining portion arranged on a peripheral wall thereof and corresponding to each of the at least one engaging portion of the first casing. Thereby, the plural tool boxes with the same size or the different sizes are stacked and fixed together easily and securely.