This study investigated the differences between Taiwanese experienced and preservice elementary school science teachers' content knowledge (CK) about electric circuits and their ability to predict students' preconceptions about electric circuits as an indicator of their pedagogical content knowledge (PCK). An innovative web-based recruitment and data collection technique were involved to purposeful collect samples totalling of 76 experienced teachers, 85 preservice teachers, and 438 students. Confidence assessment was used to assist in identifying alternative conceptions. Results indicated the experienced teachers possessed more acceptable CK with higher confidence than the preservice teachers. However, both groups' predictions of students' conceptions about electric circuits were not significantly different in two-thirds of the test items, and their learning experiences were the main source used to predict students' preconceptions. This study suggests establishing science teacher qualification to enhance preservice teachers' CK, and helping both preservice and inservice teachers become aware of their students' preconceptions were needed.