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Polysilicon layers are widely used in microelectronic and microelectromechanical devices. The layer's thermal parameters are very important for the design and fabrication of the device. However, little literature has investigated the polysilicon thin film's lateral thermal conductivity, especially at temperatures between 300 K and 400 K, with a thickness less than 300 nm. This work measures the lateral thermal conductivity of suspended undoped low-pressure chemical-vapor deposition (LPCVD) polysilicon layers, which are of 260 nm and 200 nm in thickness. The measurement is performed by using steady-state Joule heating and electrical-resistance thermometer in patterned platinum lines. The thermal conductivity is measured at three different temperatures. According to the measurement data, the thermal conductivity of the polysilicon thin films decreases with the reduction of the thin film's thickness. Meanwhile, the thermal conductivity is almost invariable in the range from 300 K to 400 K.