Abstract: A seat pillar lock device includes a seat tube having a nut, a bushing fitted into the seat tube, a seat pillar inserted into the bushing, and a lock knob fastened to the nut on the seat tube to lock the seat pillar in any of a series of elevations, and a stepped packing ring received in an expanded bottom hole on the nut and controlled by the lock knob to tighten up the seat pillar in position, the lock knob consists of a knob, a nut fixed inside the knob, a screw member having one end fastened to the knob through a toothed joint and an opposite end terminating in a threaded portion threaded into the nut on the seat tube, a latch having a screw rod at one end inserted through a longitudinal through hole on the screw member and fastened to the nut inside the knob and a latch head at an opposite end inserted through the nut on the seat tube and any of a series of vertically spaced lock holes on the seat pillar, and a tension spring stopped by the latch head inside the screw member.