An adjustment structure of a hydraulic brake system is provided, which includes a main body, a piston and an adjusting device. The main body includes at least one fluid reservoir, an inner chamber and at least one fluid passage. The inner chamber includes a cylinder channel and an opening part, and the cylinder channel is communicated with the fluid reservoir by at least one fluid port. The piston is driven by the brake cable to be limitedly moved in the cylinder channel. The piston includes a cable hole for receiving the brake cable, and the piston has a normal operated position relative to the fluid port. The adjusting device is disposed in the opening part and is abutted against the piston. The adjusting device is operated to be moved relative to the brake cable such that the piston is adjusted to be returned to the normal operated position.
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