A brake master cylinder of the invention is to dispose a main oil cylinder and an oil storing space on a handlebar base. In addition, a sleeve engaged with the main oil cylinder is disposed in the oil storing space. The one end of the oil storing space has a fastening seat passed by a draw bar linked to the brake handlebar, and a positioning sheath of the fastening seat is utilized to engage with the sleeve. A leakage prevention component is disposed between the fastening seat and the handlebar base. A bushing integrally extended from the leakage prevention component is utilized to encapsulate the positioning sheath of the fastening seat and the sleeve. Accordingly, the brake master cylinder having convenient assembly and better sealing effect for hydraulic oil is obtained.