Abstract: At millimeter-wave frequencies, corrugated horns are difficult and expensive to manufacture. As an alternative, a smooth-walled spline-profile horn constituted by sin-profile and Gaussian-profile is presented in this thesis. Its electrical property was calculated using commercially available finite element software: high frequency structure simulator (HFSS) of ANSOFT. The Gaussian coupling efficiency of the horn was calculated using quasi-optical theory. The designed horn was measured using high frequency planar near-field antenna testing system. Testing results indicate that, compared with corrugated horn, the sidelobe of smooth-walled spline-profile horn decreased 6.59 dB, its gain increased 0.5 dB, Gaussian coupling efficiency increased 0.9%, total length shortened 15.8 mm and it was much easier to manufacture. It seems that the presented horn has a high performance and it can be used as a feed source of antenna in stead of corrugated horn.