A nebulizer and a nozzle thereof are provided. The nozzle has a containing seat and a cover. The containing seat has an air conduit. The air conduit has an input terminal and an output terminal. The cover has a guiding channel and an impediment. An input opening is formed in the nozzle for introducing air. A first opening of the guiding channel is disposed near the output terminal and the input opening is disposed far of the output terminal. The impediment is disposed in front of the output terminal and has a plane. Air emitted from the output terminal lashes the plane and then flows to a second opening of the guiding channel, and the air of the input opening is guided to the first opening for decreasing a whole resistance of the nozzle, and thereby increasing nebulization efficiency.