Abstract: With the complementary antenna theory, this paper deduced the resonant frequencies of narrow slots. When the length of a vacuum slot is less than 5 times of the incident wavelength and the deviation angle of radiation relative to the perpendicular direction is less than 30°, the radiation of the slot can be explained by that of the dipole antenna, and resonances occurrs when the slot length equals to half wavelength. The dielectric slot can be equivalent to a microstrip and printed antenna with a depth half that of the slot. The theory expression matches the simulation results well with relative deviation below 5%. A further analysis of the coupling performance of the microwave-slot array indicates that the complementary antenna theory applies well in the study of microwave-slot coupling.