This study aims to (1) explore how pre-service EFL teachers implement formative assessment during teaching practicum in vocational high schools and (2) identify the challenges encountered during the implementation process. This study employed a narrative inquiry approach. The participants consisted of three pre-service EFL teachers who had completed teaching practicum in vocational high schools. The data were collected through semi-structured interviews and document analysis of lesson plans, teaching materials, and other classroom-related artifacts. The validity of the data was established through methodological triangulation and member checking. The data were analyzed using thematic analysis. The findings revealed that the participants implemented formative assessment through feed-up, feedback, and feed-forward practices. The study also found several challenges related to pedagogical and practical aspect, contextual, and personal experiential factors during the teaching practicum process. In conclusion, pre-service EFL teachers attempted to integrate formative assessment into their classroom practices during teaching practicum, although its implementation was still influenced by various pedagogical and contextual challenges. Therefore, greater support and preparation are needed to help pre-service teachers implement formative assessment more effectively in vocational high school contexts.