In this paper, a simple procedure is proposed to select the optimum design frequency for the horn antenna design by combining theory and simulation. Horn antennas have been extensively used as directive antennas at microwave frequencies. Relevant applications of horn antennas are feeds for parabolic antennas, short range radars and standard calibration antennas. Common types of horn antennas are conical and pyramidal while the most popular one is the pyramidal horn. The conventional design procedure of pyramidal horn antenna just recommends the design frequency to be chosen near the lower frequency of the operation bandwidth. In this contribution, we study the optimum design frequency for pyramidal horn antenna design and propose both a Matlab code and a closed form formula to obtain this frequency. The proposed formula has been evaluated over several standard horn antennas in different frequency bands. This contribution can be useful for engineers to understand and apply the optimum design frequency for antenna projects.