An intake regulating system of an engine equips with an intake regulating-device connected with a breather hose for drawing the oil vapor and the leaked gases from the crankcase of the engine, thereby to mix with outside air to feed into the engine for re-combustion. The intake regulating-device comprises a housing and a pressure-sensing valve. The housing has an air inlet hole, an air outlet hole, and a slender hole. The air outlet hole and the slender hole both are in communication with the air inlet of engine via an intake manifold for generating a pressure difference thereby to drive the pressure-sensing valve. Preferably, the intake regulating-device provides a pair of regulating knobs for adapting the pressure-sensing valve with various intake volumes of an engine.