Vocabulary learning strategies are essential in vocabulary acquisition and one particularly important strategy is word part strategy. This quasi-experimental research attempted to investigate the effects of word part strategy instruction on vocabulary knowledge among primary school students in a Thai EFL context. It also sought to explore primary school students' attitudes about the use of the word-part strategy instruction on vocabulary acquisition and development. The participants consisted of two intact classes divided into experimental and control groups. The experimental participants received thorough training on word part strategies, while the control group received no instruction on word part strategies. Three measures of word part knowledge were developed and validated before the data collection. A five-point Likert scale questionnaire was also employed to explore the experimental participants' attitudes towards implementing word part strategy instruction in vocabulary learning. Descriptive and inferential statistics were used to analyze the quantitative data. The results showed that the students who had received word part strategy instruction outperformed those in the control group, indicating a positive effect of word part strategies on vocabulary learning among primary school students. The current study also showed that primary school students were highly satisfied with the word part strategy instruction. Together, the present findings suggest the efficacy of word part strategies on vocabulary learning and development among primary school learners. Other implications relevant to pedagogical practices and suggestions for further studies are also discussed.