Abstract: A diagnostic system for monitoring internal conditions inside a fuel cell includes a pair of bipolar plates, a gas diffusion layer, a central controller, and three or more sensors. Each bipolar plate contains a fuel gas channel. The central controller includes a computing unit and a display. Each sensor has a resistor portion, a capacitor portion, a common lead, a resistor line, and a capacitor line for detecting a voltage value, a resistance value, and a capacity value. The display shows out physical information detected by these sensors for diagnosing the fuel cell. The physical information includes voltage, resistance, capacity, temperature, humidity, flow velocity, and flow rate. In which, it can detect various physical information inside the fuel cell. It can monitor the internal conditions continuously and directly. Plus, it can prolong the product life of the fuel cell.