A door closer includes casing fixedly fastened to a door frame, a hydraulic cylinder installed in the casing and holding a hydraulic fluid, a coupling block installed in one end of the hydraulic cylinder, the coupling block having a pivot shaft connected to a door, and a reciprocating mechanism coupled to the coupling block, fluid velocity regulating valves installed in the casing and controlled to adjust the velocity of the hydraulic fluid flowing in the casing, the reciprocating mechanism including a double-threaded adjustment rod for adjusting the spring power of compression springs thereof.