A door of an air conditioner for a vehicle, which can prevent damage and distortion by bending during assembly and reinforce strength due to a plate-shaped sliding door with flexibility. The door is disposed inside an air-conditioning case to adjust the degree of opening of an air passageway, and includes a door body formed in a plate shape and formed to be able to slide in the air-conditioning case, and a reinforcing part formed from at least one side of the door body in a thickness direction and extending in an axial direction, which is perpendicular to a sliding direction of the door body.