In this paper, a waveguide-type resonator with a slot is proposed to measure permittivity of thin film from resonant frequency shifting by an attached MUT(Material Under Test). The MUT on the slot shifts resonant frequency by perturbation of electromagnetic field. Amount of shifting resonance frequency is dependent on the permittivity of MUT, and that relation is obtained from numerical simulation. The measured relative permittivity of a thin film with thickness of $65{mu}m$ is 3.3492 with standard error of ${pm}0.0605$ in the frequency range of 2 GHz to 3 GHz. Also the proposed method is compared with other measuring methods such as dielectric resonator and waveguide probe systems.