A height adjustment mechanism for a chair armrest contains: an inner tube, an adjusting member, an outer tube, a driving block, a fixing member, and a resilient element. The inner tube includes a base, an accommodating groove, and an elongated hole. The adjusting member includes a guiding slot, two toothed racks, and a plurality of guide teeth, wherein each guide tooth has a first crown portion, a first root portion, and a first bottom face. The outer tube includes a support plate an orifice. The driving block includes a pressing portion, two flanges, and plural positioning teeth, wherein a number of the plural positioning teeth is less than that of the plurality of guide teeth. The fixing member includes a first segment and a second segment. The resilient element is fixed in the inner tube and includes a first end abutting against the driving block and the fixing member.