Abstract: A seat height adjusting device includes a seat post slidably received in a tube, the tube having a slot defined longitudinally therein and the seat post having a rack section defined in a periphery thereof, a C-shaped clamp member securely clamped to the tube and having an extending plate extending from each one of two end sides thereof which are connected by bolts, a pinion rotatably disposed between the two extending plates and engaged to the rack section of the seat post, a crank extending centrally through the pinion and a pin extending through the crank and the pinion such that when rotating the crank, the seat post is moved upwardly or downwardly in the tube by the pinion.