The present study, following Vygotskyan Sociocultural theory in education, and inspired by Rogoff’s conceptualization (1995, 2003) of development, aimed at conceptual development of  in-service EFL teachers. To this end, two Iranian EFL teachers with pseudonyms (Tara and Sara) were selected as participants of the study. The participating teachers were first taught the sociocultural concepts related to language, teaching, and learning taken from Johnson (2009) and Rogoff’s (2003) mediatory model of development in six workshops through dialogic mediation. The data for the study comprised two semi-structured interviews, and three video-recording of critical reflection of each teacher on their video-taped classroom behavior. The recordings and transcripts were analyzed using Hatch’s (2002) procedure for interpretive analysis. The results of the study showed that participating teachers, over a process of struggle with their past experiences, gradually replaced their old everyday concepts such as grammatical accuracy, correct samples, and teacher interruption with new scientific concepts such as grammatical apprenticeship, guided grammatical participation, and grammatical appropriation through assisted participation. The results of present study can be illuminating for teacher educators and teacher education programs which have aimed at changing the classroom practice of in-service teachers.