Abstract: A new dual-band co-plane microstrip/waveguide monopulse antenna is presented with its measured voltage standing wave ratio (VSWR) curve and radiation patterns. Based on the development of a new microstrip line array fed at the side and a novel compact slot patch antenna loaded with a pair of spirals, a dual-band co-plane monopulse antenna is realized with 29 mm height and less than 1 kg weight. The measured side-lobe level is lower than -19 dB and -22 dB in X-and K-band and the measured efficiency reaches 38%and 45% respectively, which is in a similar level of single band monopulse antenna. The measured results confirm the validity of this design and this antenna can meet the need of dual mode compound guidance system in future.