A height adjustment mechanism for armrests is revealed. The height adjustment mechanism includes a tubular base disposed with positioning holes, an outer sleeve disposed around the tubular base and having an opening arranged at a top end thereof, a strip rack mounted in the outer sleeve and disposed with an insertion slot on a lower part thereof, a pull handle mounted in the strip rack, a positioning member arranged at a lower end of the pull handle and inserted through the insertion slot of the strip rack to be locked in the positioning hole of the tubular base, a press member passed through the opening of the outer sleeve to be connected to a top end of the pull handle, and an alignment member disposed on corresponding surfaces of the strip rack and the pull handle for limiting vertical displacement of the pull handle and the strip rack.