【Purposes】 In communication systems, filters play an important role in filtering out-of-band signals. For the traditional parallel coupled line filter and clip line filter, the size is large and the parasitic pass band problems exist. 【Methods】 A small Ka band pass filter is designed by using radial stub. In this paper, the radial is analyzed, calculated, and simulated. Then, the filter is modeled and simulated with HFSS. According to the simulation results, the initial model is optimized and improved by adjusting the radius and angle of the radial stub. Finally, the filter is processed by using RO5880 substrates. 【Findings】 The actual test results show that the center frequency of the filter is about 36.9 GHz and the bandwidth is about 4.57 GHz, which reashes the parameter requirements. 【Conclusions】 Compared with parallel coupled line filters, this design occupies less volume in the same frequency band, and has better suppression effect on out-of-band signals in the low frequency band.