This study presents a novel composite right/left-handed (CRLH) leaky wave antenna (LWA) with horizontal plane scanning capability. This LWA consists of 10 CRLH unit cells and coplanar waveguide to coplanar stripline transitions. The proposed balanced CRLH unit cell is based on an asymmetric coplanar stripline structure (ACPS). The parameters of the CRLH unit cell can be determined analytically, enabling easy modification for any other frequency band. Because of its planar and horizontal plane scanning property, the proposed design can also be extended to a sector-shaped multisection LWA, achieving a scanning range of more than 180° in the horizontal plane. By varying the operating frequency, the CRLH leaky wave antenna shows the continuously beam-scanning property from backward to forward in the horizontal plane. A 10-cell prototype exhibits the beam-scanning capability in the horizontal plane, and a 20-cell multisection sector-shaped LWA confirms the approach of designing the wide-range beam-scanning LWA. Experimental results show that the proposed design is in good agreement with the simulation. The planar and low-profile characteristics of the novel CRLH LWA make it suitable for integration with other microwave circuits, and can be applied to applications that require a wider beam-scanning range.