This letter presents the design of a new omnidirectional antenna, which is composed of annular slot and annular patch stacked together for dual-band operation at 2.5 and 3.5 GHz. The influence of the antenna parameters on the dual-band operation is given with simulations. A prototype of the proposed antenna was manufactured with FR4 substrates for the purpose of demonstration. The measured reflection coefficient exhibits dual-band operation at 2.32 GHz with fractional bandwidth of 19.3%, and at 3.59 GHz with fractional bandwidth of 18.6%. The stable measured omnidirectional radiation patterns can be obtained at both frequencies. The agreement between the measured and simulated patterns can be observed from the experimental results.