A three-dimensional finite difference time domain (3-D FDTD) algorithm of calculation for the characteristics of coaxial-to-cavity filter including a discended probe and a tuning post is presented. In the arrangement the inner coaxial conductor protrudes into the cavity and operates as a probe radiator. And the height of the rectangular tuning post determines the resonant frequency. The validation of this method is confirmed by comparing the calculated values with experimental results for the designed filter.