A novel temperature difference-to-frequency converter using two resistance temperature detectors (RTD) has been developed. The resistance difference of two RTD is converted into its equivalent inductance to form the resonant circuit of the Colpitts oscillator. The conversion sensitivity of 16 Hz/$^{circ}C$ and the residual nonlinearity less than 2.15% over the temperature difference range from 35$^{circ}C$ to 155$^{circ}C$ are obtained by the prototype converter. The frequency drift of oscillator itself is $pm$ 0.5Hz. Thus, the minimum detectable temperature difference is estimated to be $pm$ 0.013$^{circ}C$. The proposed coverter, except for two RTD, can be fabricated in monolithic IC form.