Abstract: A non-return valve includes a flap and a valve body. The flap has one side formed with a pivot connector pivotally connected with the valve body and the other side formed integral with a weight. After the flap is pushed open by the pushing force of fluid and the pushing force of the fluid vanishes, the flap without being pressed by any force will be actuated by its own weight to quickly move down and tightly close the valve port of the valve body, having excellent effect of stopping fluid from flowing reversely and leaking out of the port.