A hidden hydraulic structure of a bike disc brake is integrally disposed with a bike body tube. The hidden hydraulic structure of the bike disc brake includes a base, a piston and a linking member. The base includes an oil circuit. The piston is located in the base for controlling a pressure of the oil circuit. The piston is actuated by the linking member. Wherein a shape of the base is corresponded to a shape of the bike body tube, and the base is connected with the bike body tube to form a portion of the bike body tube, thereby increasing strength of the bike body tube.