The purpose of the current study was to investigate whether and to what extent formative assessment influenced the EFL learners' vocabulary achievement. It also aimed at determining the students' attitudes towards using formative assessment in vocabulary learning. The research was conducted at the American University of Armenia. Two groups were involved in the research, the experimental and the comparison. The experimental group practiced the vocabulary with the help of formative assessment, whereas for the comparison group traditional exercises and activities were implemented. Both qualitative and quantitative data were employed for the research. The instruments applied were pre- and post-tests, an attitudinal questionnaire and a semi-structured interview. The analysis of the pre- and post-tests showed a significant difference between the two groups. The results of the questionnaire and the interview revealed that formative assessment had positive influence on the learners' vocabulary enhancement. The students claimed that formative assessment techniques helped them to become actively involved in their learning and motivated them to learn the English vocabulary.