A microstrip slit-tetragonal resonator gap-loaded with a piece of silicon is proposed as optical tuning element. Change of photoconductivity of silicon, after irradiation of it with an infrared lamp, results a significant change of density (25 dB) of the first resonant peak (n = 1/2) of resonator. This behavior makes this device a possible candidate for optical tuning of microwave transmission lines at specific operating frequency. Measured S-parameters of the proposed device are in good agreement with simulated ones found from finite difference time domain method. Calculation of S-parameters based on a transmission line model is included for comparison.