In this action research study, the researcher aimed to find and address a problem that English as a Foreign Language (EFL) learners in her class come across to improve their language learning process. The action research study adopted mixed methods design using both qualitative and quantitative data collection and analysis methods. Firstly, to identify the problem, the researcher observed the students closely, analyzed their midterm scores, and administered a diagnostic survey to the students. The data collected through these procedures indicated that vocabulary is the most problematic area for learners. Then, to diagnose the source of the problem, the researcher used a think-aloud protocol. In this way, rich verbal data about students’ reasoning during a vocabulary task were collected. After identifying that the source of the problem was a lack of strategy use, strategy training for contextual clues was implemented as an intervention for five weeks. Finally, the effectiveness of the intervention was evaluated through a paired-samples t-test. The analysis showed that students had significantly higher results in the vocabulary section of the second midterm compared to the first midterm. This study found that strategy training for using contextual clues is effective in improving EFL learners’ vocabulary mastery, independent of their language level.