Vocabulary plays an important role in the Content-Based classroom, making EFL Students' science vocabulary knowledge critical to their content understanding. The purpose of this action research was to enhance students' science vocabulary knowledge of 9th grade students through science vocabulary crossword puzzle (SVCP) practices. The research group comprised 33 students of a science classroom using English as the medium of instruction in their second semester of the 2020 academic year at a large secondary school in Bangkok. The action research cyclical process of Kemmisand McTaggart (1981) was adopted through the steps of (1) Plan, (2) Act, (3) Observe and (4) Reflect. The innovation being introduced was a collection of science vocabulary crossword puzzle materials. The research instruments used were the science vocabulary knowledge test and the students' opinion questionnaire towards SVCP practices. The assessment of students' vocabulary knowledge was conducted before and after the delivery of crossword puzzle practices. The data was analyzed by a dependent sample t-test. The research findings were as follows: (1) After implementing SVCP practices, the mean of students' science vocabulary knowledge score had increased by a statistically significant 0.05. and (2) students had positive opinions toward the SVCP practices.