Abstract: The design of a novel planar dual-band(Ku/Ka) and dual circularly polarized slot antenna is proposed in this paper. Three parallel plates compose a two-fold radial line waveguide, the top of which is an aperture with slots. Slot pairs of two bands are arrayed in counterclockwise and clockwise spiral to achieve Right-hand and left-hand circularly polarized radiation respectively. Different feeding structures are used, which one central probe for the lower band while multi-probes for the higher band. We add a ring of metal vias on the periphery of the radial line waveguide of higher band, which makes it easier to achieve good impedance matching at lower frequency band. The simulation results show that, at Ku and Ka band, the highest radiation efficiency is 60.5 and 55.7 percent, the -1 dB bandwidth about gain is 8.3% and 6.2%. The antenna has broad axis ratio bandwidth and excellent port isolation. The antenna has good engineering application value.