Early childhood teachers are critical in promoting young children's STEM learning. However, research indicates that early childhood teachers are unfamiliar with STEM and do not have confidence in their abilities and skills to implement STEM education. Despite this issue, previous research has focused less on early childhood teachers' STEM professional development. This mixed-method study investigated the effects of the STEM Professional Development Program (SPDP) on 12 early childhood teachers' attitudes toward STEM education, STEM practices, self-efficacy, and instructional beliefs, which were used as pretest and posttest measures. The quantitative dimension of the study investigated the effects of the SPDP on early childhood teachers' attitudes and STEM practices, self-efficacy, and instructional beliefs towards STEM education by using the pretest and posttest only, without a control group design. The qualitative dimension, participating teachers' experiences, perceptions about the SPDP, and thoughts about the SPDP's influence on their beliefs, self-efficacy, and attitudes about STEM were explored using phenomenology design. Quantitative data were analyzed using the Wilcoxon signed-rank test. Qualitative data were analyzed using content analysis. Results showed that the SPDP positively affected teachers' self-efficacy for STEM practices, attitudes toward STEM education, and instructional beliefs. Moreover, participating in the SPDP influenced teachers' perspectives on STEM education, supported their learning about STEM content, and increased their awareness of the importance of STEM in early childhood.