A composite right/left-handed (CRLH) leaky wave antenna (LWA) using double mushroom-like structures is proposed. With a proper arrangement of the left-handed structures, desirable cross-polarization performance in two orthogonal planes can be obtained based on the differential excitation principle. The CRLH performance of the cascaded LWA is demonstrated, and its improved radiation performance is clarified. Measured results indicate that the proposed antenna operates in 9.7-16.4 GHz with a beam scanning range from -71° to +31°. The cross-polarization levels are less than -30 dB and -20 dB in the beam scanning plane and non-beam-scanning plane, respectively.