This study investigated the influence of Engineering Is Elementary (EiE) professional development on teachers' self-efficacy of integrating engineering into the K-5 curriculum in a rural school district in southeastern North Carolina. In fall 2016, the researchers conducted an embedded mixed-method study. The focus of this paper is the quantitative aspect of the study, which involved using the engineering components of the T-STEM survey to measure teachers' self-efficacy via Qualtrics. The survey was used to compare teachers' self-efficacy before and following EiE professional development and 4 weeks after the last EiE intervention. Forty-three teachers completed these online questionnaires. Across the three intervals, the results of the repeated measures were statistically significant. There were increases in teachers' (a) engineering teaching efficacy and beliefs, (b) engineering teaching outcome expectancy, and (c) engineering instruction. Teachers' self-efficacy toward engineering was likely influenced by EiE professional development. The findings suggest that elementary teachers' self-efficacy about integrating engineering into the curriculum can increase by offering EiE professional development over time. This study can help inform future education policy, practice, and research.