A rail structure for optical fiber cassette is adapted to be assembled on a board member of a fiber box. The first engaging structure and the second engaging structure at two ends of the rail body are movably engaged with the first buckling structure and the second buckling structure. Hence, the user may detach one of the rail structures from the board member, so that the distance between the rest two of the rail structures goes wider, and the user can assemble another optical fiber cassette between the two rail structures, and the type and the size of the optical fiber cassette may be different from the detached one. Hence, the rail structure has wide applicability to allow the user to change the optical fiber cassette with different types and specifications conveniently and quickly.