Abstract-Fire-resistant turbine oil is widely used in thermal power plant. As the service time increases, its lubrication and electrical characteristic degrade. This work proposes a mini NMR (nuclear magnetic resonance) sensor for assessing the degradation of the turbine oils used in real time and in vivo. Two magnetic discs with poles opposed were separated in a distance to generate a relative homogeneous static magnetic field between them. A solenoid coil was placed between the magnets as the RF coil. The dimensions of the sensor are 30 mm × 30 mm × 36 mm and the mass is 107 g. The T 1 and T 2eff of the turbine oils from two different power plants were measured. The results demonstrated that an increase of the service time of turbine oils clearly results in a decrease of T 1 and T 2eff . This method of monitoring degradation of turbine oils could lead to the use of NMR to assess the degradation level of turbine oils in service.