The purposes of this study are to report the influences of a mixed delivery professional development [PD] course involving face-to-face classes and the mentoring assisted inquiry-based teaching [MAIT] website that addressed the conceptual change and self-efficacy of high school mathematics and science teachers' conceptions of inquiry-based teaching. Twenty-five in-service high school science and mathematics teachers participated in a 9 weeks, 4 h per week PD course. Data collection included the pre-post Inquiry Teaching Efficacy Questionnaire [ITEQ], teachers' reflective journals posted on the MAIT website, teachers' assignments requested by the course instructor, and follow-up teachers' interviews. Findings revealed that both mathematics and science teachers' conceptions of inquiry teaching show significant improvements after the course. The mixed delivery PD course had the same influence on science and mathematics teachers. The authentic video clips and lesson plans provided on the MAIT website and the specific feedback toward teachers' performance are the main factors that shaped teachers' inquiry-based teaching conceptions.