The purpose of this paper is to explore the relationship between preservice teachers' (PTs) teacher efficacy beliefs and their constructivist-based teaching practices. Data were gathered through the questionnaire (Teachers' Sense of Efficacy Scale) and the observation protocol (Reformed Teaching Observation Protocol) administered to the participants. A total number of 101 PTs (53.5% from science education and 46.5% from mathematics education) from a university in eastern part of Turkey participated in the study. Also, qualitative data were also used in order to clarify quantitative data. The semistructured interviews were conducted with 20 PTs who voluntarily participated in these interviews. Results showed that PTs' constructivist-based teaching practice was positively correlated with their teacher efficacy beliefs. Also, qualitative findings confirmed that finding. To conclude, PTs with high teacher efficacy tend to employ constructivist approach in their teaching while PTs with low teacher efficacy tend to use traditional approach, lecturing in their teaching.