A novel ring-shaped MMW active monopulse antenna suited for MMW/IR dual-band radar application is proposed. The microstrip array antenna including four subarrays has been chosen. Each subarray is connected with a separate T/R making a four-element active antenna. The merit of increasing the power transmitted and the receiver's sensitivity make the gain both the transmitting and receiving antenna higher, which make the angle measurement system's performance better. The radiation patterns of the active antenna was measured and the results show its superiority compared to the traditional one. The experiment was done for angle measurement using the active system. The results showed its validity and practicability.