Based on the principle that the step impedance resonator (SIR) is smaller than the uniform impedance resonator (UIR) at the same frequency, a miniaturized interdigital filter is designed and fabricated by using thin film technology in this paper. The length of SIR resonator is 248% shorter than that of UIR resonator. A miniaturized interdigital filter is designed using this miniaturized resonator. The filter is fabricated on a 0254 mm thick ceramic substrate with a relative dielectric constant of 98 using a thin film process. The measurement results show that the passband of the filter is 355~435 GHz, the insertion loss at the center frequency is 3 dB, the flatness in the band is 17 dB, the return loss in the band is less than -186 dB, and the rejection at 28 and 51 GHz outside the band is -428 and -661 dB respectively. The size of the miniaturized filter is only 640 m×479 mm（021λg×016λg）.