A seat tube of a bicycle has a tubular body and a tightening block. The tubular body has an insertion hole and a stationary block. The insertion hole is defined in the tubular body and has an inner surface and a tightening recess. The inner surface is formed in the tubular body. The tightening recess is defined in the inner surface. The stationary block is embedded in the tightening recess and has an oblique surface and a fastener hole. The fastener hole is defined in the oblique surface. The tightening block is mounted slidably on the oblique surface and has a tightening surface, a through hole and a fastener. The tightening surface is shaped to correspond to the inner surface. The through hole is defined through the tightening block. The fastener is mounted through the through hole and engages the fastener hole.