Abstract: A mechanical disc brake including an inner caliper, an outer caliper and a brake lever is disclosed. The inner and outer calipers are associated with each other by bolts and fixed on a bicycle by a U-shaped bracket. The inner caliper is formed with circular hole and two lateral faces of the circular hole are expanded to form circular dents. One of the circular dents is formed with recessed spiral threads and a transmission piston thread rod formed with projecting spiral threads is fitted in the circular dent. The transmission piston thread rod is sequentially connected with an adjustment thread rod and a lining assembly. The outer caliper is correspondingly disposed with a lining assembly. A restoring spring is disposed in the other of the circular dents and connected with a brake lever. When the brake lever is pulled, the transmission piston thread rod, adjustment thread rod and lining assemblies are pushed forward, whereby the lining assemblies cooperatively tightly clamp the brake disc.