This study examined the influence of expressive writing on working memory capacity (WMC), a component of executive function. Japanese undergraduates (N = 104) were individually tested across six separate experimental sessions (baseline, three writing, and two follow-up sessions at 1 and 5 weeks). Participants were randomly assigned to write about a traumatic experience, their best possible future selves (BPS), or a trivial topic for 20 minutes. WMC tests were completed at baseline and follow-ups. Results indicate that expressive writing about traumatic experience can improve WMC at 5 weeks after writing, but that writing about BPS or trivial topics has no effect on WMC.