An air inlet is mounted in a baby bottle and has a valve and an air-inlet tube. The valve has a body, an outer ring and an inner ring. The body has a chamber, multiple separating blocks and multiple liquid channels. The separating blocks are formed on the body and each has a vent. The liquid channels are formed through the body between the separating blocks and communicate with the chamber. The outer ring is formed around the chamber to separate the chamber and the vents. The inner ring is formed on an inner end of the body around the vents and has multiple air inlets. The air-inlet tube is mounted detachably on the inner ring of the valve. Therefore, liquid flows through the liquid channels and the chamber, but air flows through the air inlets and the vents to the air-inlet tube to ensure smooth operation.