The purpose of the research reported here is to discuss teachers’ learning and perceptionalchange/s of ELT teachers and to be able to find out the consistency between conceptual andbehavioural changes of teachers from the view of constructivist approach. The relationships ofpersonal theories (constructs) and theories in action (behaviours) will be dealt with. Furthermore, thecontent and structural changes of constructs between the beginning and end of the study will bediscussed. The nature of the constructs studied here will be presented through the use of multiple datasources-but with special reference to repertory grid data-so that a more comprehensive understandingof the participant’s thinking system is possible. We will exemplify the change by considering theconstructs of an ELT teacher. In this study, we have presented the participant’s personal theoriesdeeply and from various perspectives during an in-service teacher development program.