A compact dual-band rat-race coupler is proposed in this paper. By using composite right/left-handed (CRLH) transmission lines (TLs) and investigating all possible combinations of phase responses of the individual TLs in the coupler, a dual-band and miniaturized rat-race coupler was implemented. The dual-band rat-race coupler shows a 55% size reduction. This CRLH-based coupler is used as a mode-decoupling network in a dual-band front-end MIMO system, along with a planar antenna array, to split two orthogonal radiation modes from the connected array. A pair of compact dual-band antennas were built and closely spaced to demonstrate pattern diversity by in-phase or out-of-phase excitations from the coupler. Good experimental isolation of the system, -29 dB at 2.4 GHz and -34.2 dB at 5.2 GHz, is exhibited verifying the desired decoupling property. Furthermore, pattern diversity in MIMO communications was observed by two measured orthogonal radiation patterns.