A fitting structure for tools contains: a fitting end of a fitting tool and a fitted end of a fitted tool. The fitting end includes a toothed retaining area formed on an inner wall thereof, a concaved portion defined on an upper end thereof, a peripheral groove arranged around the concaved portion, and a ring disposed in the peripheral groove. The fitted tool includes a working portion arranged on a lower end thereof, a polygonal engaging portion defined over the working portion, an annular locking portion formed over the polygonal engaging portion. The annular locking portion has an annular lock trough defined thereon, and the annular lock trough has a color marking element fixed therein for identifying the fitted tool.