Abstract: A height adjustment mechanism is mounted under a chair or table. The mechanism comprises an adjustment lever extended from the top; a support tube having a longitudinal hole with a predetermined cross-section; a sliding tube with the predetermined cross-section and having an upper end coupled to the end of the lever, a lower end slidingly received in the longitudinal hole of the support tube, a longitudinal hole, and an aperture on the surface; a bar slidingly received in the sliding tube; and a sleeve having a spring depressible detent on the surface and a longitudinal slit such that the sleeve sandwiched between the bar and the sliding tube is flexible for providing a buffering; wherein the lever is operable to cause the spring depressible detent to fall into the aperture without turning chair or table while adjusting the height thereof.