An adjustment structure which is used in a hydraulic brake system and is coordinated with a brake cable controlled by a brake lever , the adjustment structure comprising: a main body , comprising a fluid reservoir , an inner chamber and at least one fluid port , wherein the inner chamber comprises a cylinder channel and an opening part communicated with the cylinder channel ; an adjusting device screwed in the opening part of the main body , wherein the adjusting device is rotated so as to move reciprocatedly in the cylinder channel , and the adjusting device comprises an annular fluid guiding passage , wherein the annular fluid guiding passage comprises: a guiding port ; and an annular groove connected to the guiding port , wherein the annular groove surrounds an outside of the adjusting device and is communicated with the cylinder channel and the fluid reservoir via the at least one fluid port ; a piston connected to the brake cable and liquid-tightly disposed in the adjusting device , wherein the piston is limitedly moved in the cylinder channel and comprises a cable hole for receiving the brake cable , and the adjusting device is rotated to adjust the piston to be located at a normal operated position (B) relative to the guiding port ; a returning spring abutted between the main body and the piston ; characterized by a positioning device disposed at the main body , wherein the positioning device comprises an abutted ball abutted against the adjusting device ; wherein the adjusting device comprises a positioning groove coordinated with the abutted ball . a main body , comprising a fluid reservoir , an inner chamber and at least one fluid port , wherein the inner chamber comprises a cylinder channel and an opening part communicated with the cylinder channel ; an adjusting device screwed in the opening part of the main body , wherein the adjusting device is rotated so as to move reciprocatedly in the cylinder channel , and the adjusting device comprises an annular fluid guiding passage , wherein the annular fluid guiding passage comprises: a guiding port ; and an annular groove connected to the guiding port , wherein the annular groove surrounds an outside of the adjusting device and is communicated with the cylinder channel and the fluid reservoir via the at least one a guiding port ; and an annular groove connected to the guiding port , wherein the annular groove surrounds an outside of the adjusting device and is communicated with the cylinder channel and the fluid reservoir via the at least one fluid port ; a piston connected to the brake cable and liquid-tightly disposed in the adjusting device , wherein the piston is limitedly moved in the cylinder channel and comprises a cable hole for receiving the brake cable , and the adjusting device is rotated to adjust the piston to be located at a normal operated position (B) relative to the guiding port ; a returning spring abutted between the main body and the piston ; characterized by a positioning device disposed at the main body , wherein the positioning device comprises an abutted ball abutted against the adjusting device ; wherein the adjusting device comprises a positioning groove coordinated with the abutted ball . <EOS>