A design of dual-band cavity-backed slot antenna is presented for GPS and S-band radar applications. In the design, a very thin cavity (0.017 λ0 in thickness at 1.575 GHz) is used to achieve unidirectional radiation. The dual-band responses of the antenna are excited by the slot resonance at 1.575 GHz and waveguide transition effect at 2.4 GHz. Even with very small cavity thickness, reasonable impedance bandwidths are obtained as 1.6% (26 MHz) at 1.575 GHz and 8.4% (203 MHz) at 2.4 GHz. The proposed slot can also be loaded with a spurline to adjust the antenna centre frequency and the phase of the radiated field. All designs in this paper have been validated with experiments. Satisfactory results have been obtained.