A tire pressure warning assembly includes a tire valve with a switch received therein, which had a conductive member, a movable member and a biasing member. The movable member is located at a distance from the conductive member and one end of the movable member is exposed to the air in the valve and the other end of the movable member is in contact with the biasing member, which exerts a force to the movable member. The tire pressure compresses the biasing member. When the tire pressure is less than the biasing force of the biasing member, the biasing member pushes the movable member to contact the conductive member to form a circuit so that the warning device sends a signal to the driver.