Abstract: An air ventilator for a helmet that can enhance the assemblability and stability by improving the assembly structure of the air duct assembled to the top of the helmet, is disclosed. The air ventilator comprises an air duct 20, a guide rail 40, and a slider 50. The air duct 20 is assembled on the top surface of the helmet 1 so as to cover around a plurality of air ventilation holes 1a. The guide rail 40 has both sides bent upward widthwise and bent again in the direction of the edge of the width to form first flanges 41. The slider 50 has both sides bent upward widthwise and bent again in the direction of the center of the width to form second flanges 51 and slidably mount on the guide rail 40 in close contact with the bottom of the air duct 20.