This study investigated the association between seductive details effect and level of learning vocabulary. An online experiment targeting participants aged 18–29 was conducted in April 2018 (n＝936; 407 men and 529 women). Participants were randomly divided into two groups and provided an educational session on English vocabulary. Only one group was presented seductive details (i.e., a stopwatch and number of learners currently online). KATAGIRI’s (2002) 10-minute vocabulary test was administered before and after the session, and scores were compared across groups. The results showed that seductive details helped lower-level learners, which was mediated by their time spent in learning. In other words, learning seems to be expedited by seductive details among lower-level learners. However, seductive details inhibited higher-level learners. It was interpreted that seductive details inhibited metacognitive monitoring in higher-level, which they often use as a learning strategy.