Seventh graders from a STEM school performed poorly with rational number concepts in the yearly standardized test. This research investigated the impact of a three-day professional development intervention focused on improving math teachers' self-efficacy when teaching rational numbers. The study employed a concurrent mixed-method research design to understand the outcomes comprehensively. Six mathematics teachers participated in a three-day professional development program about reinforcing their teaching practices and using virtual manipulatives to increase their self-efficacy. To collect pre- and post-data on teachers' self-efficacy, a 25-item Likert scale, Mathematics Teaching Efficacy Belief Instrument by Riggs and Enoch (1990) was used. It distinguishes two dimensions of efficacy beliefs for mathematics teachers: personal mathematics teaching efficacy and mathematics teaching outcome expectancy. The researcher conducted focus group interviews and took field notes about teachers' expressions and behaviors to examine participants' in-depth insights, perceptions, and experiences. A descriptive statistics data analysis found an increase of 0.5 in the overall test means between pre- and post-means, 10% of 5 on the Likert scale, showing that teachers' self-efficacy increased after the PD. Also, the findings revealed that the PD intervention positively influenced math teachers' self-efficacy. The teachers showed a shift in motivation and involvement, believing their mathematical abilities could be developed through practice, perseverance, and effective strategies. Teachers participating in relevant PD may gain confidence in teaching math effectively, resulting in better classroom practices. The outcomes of this study were shared to help guide the development of targeted interventions to support the continuous professional growth of mathematics educators. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]