We investigated the effect of the variability of time interval between a prime and a target on reaction times and event-related brain potentials (ERPs) in a lexical decision task. In Experiment I, thirty six graduates and undergraduates read silently a prime, and classified a target into word or non-word. The stimulus onset asynchrony (SOA) between the prime and the target was 500, 1000, or 2000 ms. For a half of participants, SOA was fixed within a experimental block, while three SOA conditions were randomly mixed within a block for another half of participants. The results show that the reaction times were shorter when the target was semantically related to the prime than when the former was unrelated to the latter. The degree of this semantic priming effect was not influenced by the variability of SOA. In Experiment II, we recorded scalp ERPs from 20 graduates and undergraduates who participated in the same task as Experiment I. The results of reaction times showed a replication of Experiment I. Two negative deflections of ERPs (N375 and N450) sensitive to semantic relation between prime and target were identified. Neither the amplitude nor the latency of peaks of these deflection changed according to the variability of SOA. These results suggested that a temporal aspect of controlled attention would not affect the enhancement of selectivity based on semantic information.