According to the 'Regulations of the Automobile Transportation' of Taiwan, any passenger on the bus who is over 4 years old must fasten seat belts when the bus is driving on the free high way. To enforce passengers to follow the regulation, this study designs and implements a smart seat belt warning system by which the driver of the bus can detect whether each passage fastens the seat belt and the detection result will be displayed on the smart cell phone. The implemented achievement includes a new seat belt buckle which includes a WIFI module and a designed APP. It only uses one WIFI module, and rewrite the module into STA mode. As long as the seat belt buckle is locked in, the WIFI modules on all seats can be connected such that the bus driver can know the belt of the seat is fastened from the APP of the cell phone. Otherwise, if any seat belt is not fastened, the driver also can find it. The advantages of the product are that it is very cheap and the original seat structure does not need to be changed.