A tool box assembly includes a main case having two main casing halves, each of which has top, bottom and two opposite side ends. The main casing halves are hinged together at the bottom ends. Each of the main casing halves has a wall panel which extends between the top, bottom and side ends and which is opposite to the wall panel of the other one of the main casing halves. The wall panel has an inner part, which is formed integrally with an inner containing portion, and an outer part which has an outer face and which is formed integrally with an outer containing portion. Two subcases are respectively and removably received in the outer containing portions of the main casing halves. Each of the subcases has a cover and a housing which is hinged to the cover and which contacts face-to-face the outer face of a respective one of the main casing halves. Interlocking members are used for releasably retaining the subcases in the outer containing portions, and are formed integrally on the wall panels of the main case and the housings of the subcases.