In automotive applications, magnetic field sensors are widely used for detecting position and current. However, magnetic field sensors are required to be highly precise with good usability. To satisfy demand, we have developed a graphene Hall sensor that senses magnetic fields by the Hall effect. The sensitivity of a Hall sensor is proportional to the carrier mobility, and graphene has an extremely high carrier mobility compared with conventional materials like Si, GaAs and InSb. Thus, graphene Hall sensors are expected to give high sensitivity that will enable sensing of the Earth’s magnetic field. In addition, graphene has a low temperature dependence on carrier mobility due to its ballistic transport, so good usability in actual use is also anticipated. In this paper, we demonstrate a graphene Hall sensor made using conventional Si process technology. Our devices exhibit a sensitivity of 0.1 V/VT with a mobility of about 2,000 cm2/Vs, and a thermal coefficient of sensitivity (TCS) of 2800 ppm/K. This data indicates the possibility of constructing a graphene Hall sensor for practical use. We also show practical data such as the temperature dependence as the operation mode changes between constant-current and constant-voltage modes.