This letter presents a new low side-lobe horn antenna with nonuniform slot array operated at K and Ka band. In the proposed antenna, the nonuniform slot array reduces the side-lobe level by a modification of electric-field distribution in horn aperture. The size of the proposed horn antenna is 78 x 75 x 400 mm3, and the experimental results exhibit a peak gain of 25-28.7 dBi and side-lobe level of about -20 dB in target band.