An apparatus of measuring concentration of fuel including a catalyst layer, a diffusion layer, a fuel chamber, a reactive gas chamber, and a sensor is provided. The diffusion layer is connected to the catalyst layer. The fuel chamber is suitable for containing a fuel. The diffusion layer is between the fuel chamber and the catalyst layer. The reactive gas chamber is suitable for containing a reactive gas. The catalyst layer is between the reactive gas chamber and the diffusion layer. The fuel diffuses to the catalyst layer via the diffusion layer such that a combustion reaction of the fuel and the reactive gas is conducted in the catalyst layer to consume the reactive gas and generate a gaseous product. The sensor is disposed on the reactive gas chamber for measuring the concentration of the reactive gas or the concentration of the gaseous product in the reactive gas chamber.