This paper presents an optically controlled coupled microstripline microwave powerattenuator on a high resistivity silicon substrate in the 3GHz-7GHz frequency band. The structure is based on the10dB microstrip directional coupler. Attenuation control is performed by optical control of the open end resistances. The open end act as a variable resistance under sufficient optical illumination conditions. The attenuator has been designed for 10dB and 25dB attenuation control for one-port and two port optical illuminations respectively. The analysis has been carried out for one port illumination using a 30mW, 850nm laser diode and two ports illumination using two 20mW, 650nm laser diode. The attenuator has been modeled by the Computer Simulation Technique's Micro Wave Studio (CST MWS) 3D EM simulator. At 5GHz, the structure provides a continuous variation of S41 between 0 and 25dB with both open end illumination.