According to the race models of the stop-signal paradigm, stopping success (successful vs. unsuccessful stopping) is attributed to the finishing times of a go and a stop process. In addition to those factors involving processing times, in the present study we sought to use electrophysiological measures to find factors involving activations that could affect stopping success. We hypothesized that voluntarily-generated unimanual preparation would be a factor. To assess voluntarily-generated unimanual preparation in the stop-signal paradigm, we used a selective-stopping task without any precue. The selective-stopping task also allowed us to assess reaction times (RTs) even when stopping was successful. We demonstrated shorter RTs in signal-respond (i.e., unsuccessful stopping) than in signal-inhibit (successful stopping) trials, as is predicted by the race models. More importantly, we also demonstrated different pre-signal lateralized readiness potentials between the two types of trials and larger lateralized mu ERD in signal-respond than in signal-inhibit trials, suggesting that voluntarily-generated unimanual preparation affects stopping success. In addition to what is described in the race models of the stop-signal paradigm, the present results therefore demonstrated measures of pre-signal activations that could influence stopping success.