The purpose of this study was to probe the differences of perceived professional teaching competence between elementary school math/science teachers in Taiwan who are majored in math/science and those who are not. A researcher-developed Math/Science Teachers' Professional Development Questionnaire was used in a nationwide survey, using a two-stage stratified random sampling involving 556 elementary schools and 1374 math/science elementary school teachers. The original questionnaire consisted of a total of 105 Likert scale questions distributed unevenly among four subscales. In this study, the authors selected a subset of 63 items from the above questionnaire and analyzed the available data in order to find out whether there is any significant difference in the perceived professional teaching competencies between the two groups of elementary school math/science teachers. The internal reliability of the selected items as measured by Cronbach's alpha was 0.97, while the result of a factor analysis indicated that the selected subset of items contained 10 key factors. ANOVA tests were conducted subsequently, and the results indicated that there were 9 key factors in which significant difference between the two groups of teachers existed. Considering their practical significance, two factors focusing respectively on teachers' self-efficacy in inquiry skills and abilities to provide students a learning environment that helps them understand the nature of math/science were identified as areas in which professional competencies for non-math/science majors need to be strengthened.