Vocabulary is at the surface level of language usage; thus, students need to develop mathematical and scientific vocabulary to be able to explicitly communicate their mathematical and scientific reasoning with others. The National Council of Teachers of Mathematics (NCTM) and the National Science Teachers Association (NSTA) have both created communication standards within mathematics and science disciplines. In the present study, science, technology, engineering, and mathematics (STEM) project based learning (PBL) methods were utilized during a summer camp in 2013 to encourage interest in and grow the knowledge of students in the STEM disciplines. The participants (N = 53; 18 female, 35 male, 5 Asian, 6 African American, 12 White, and 30 Hispanic) were 8th grade students. The paired-sample "t" tests' results showed that the model of STEM PBL instruction elicited a statistically significant (p < 0.05) improvement in the mathematical and scientific vocabulary knowledge of students with the "Cohen's d" effect size of 0.62 and 0.84 respectively. STEM PBL could be a beneficial instructional method concerning vocabulary mastery for students in science and mathematics classes.