In this paper, a 6-10-GHz ultrawideband (UWB) directional reduced-size wide slot antenna (RWSA) for on-body wireless communications is proposed and investigated. The RWSA is designed to be narrow in width and can be mounted on the side of a wireless device. When a side-mounted RWSA is worn on a body, the antenna boresight is naturally directed along the body surface, and most of the energy can be efficiently transmitted by direct or diffracted waves. Consequently, the on-body channel performance can be effectively improved. After a series of on-body channel measurements, the results show that, with the side-mounted RWSAs, the path loss is significantly improved (greater than 20 dB) compared with the results of previous studies, and the sensitivity of the channel to the human body and environment is effectively lessened. An approach to estimate the variation range of path loss in real environments is also proposed at the end of this paper.