This study examined the effectiveness of a brief theory-based intervention on muscular strength among adolescents in a physical education setting. The intervention adopted a process-based mental simulation technique. The self-reported frequency of practising for and actual levels of abdominal muscular strength/endurance as one component of health-related physical fitness was targeted. Adolescents aged 16–19 years (n = 105) from nine classes were cluster-randomly assigned to experimental, mere-measurement control, and no-measurement control groups. All participants performed the one-minute sit-up test at baseline and four weeks later to measure their abdominal muscular strength/endurance. In the middle of the four-week period, participants allocated to the experimental group mentally simulated the process (i.e. practising) for doing better on the follow-up test relative to the baseline. Results revealed a significant effect of the process mental simulation on test performance at follow-up, but no effect on self-reported frequency of practising for the follow-up test. There was no evidence for the mediation of the effect of process mental simulation on test performance by planning, motivation, or anxiety variables. Results supported the effectiveness of the process mental simulation intervention in enhancing muscular strength among adolescents in a physical education setting, but not in promoting frequency of practise. The latter may be due to the process mental simulation not increasing the frequency but rather the quality of practising. This knowledge could help physical education teachers to facilitate adolescents’ endeavours in improving their muscular strength as one component of health-related physical fitness.