A damping device comprises an axle, a housing, a unidirectional member, and a damping oil. The unidirectional member in cross section is formed like a T-shape, and a top portion thereof is formed as an arc to cooperate with the housing's inner wall. Where the unidirectional member contains a U-shaped slot is defined oil holes interlaced with the gap of the first slot wall. The lifting of the cover generates little damping and the closing thereof generates a variable and yet optimal damping. The undersized unidirectional member also adds strength to the structure. The diminished cross-section of the unidirectional member contributes to a smaller U-shaped slot.