The aim of this study is to determine the effect of a project on teachers' self-efficacy beliefs about organizing trips to out-of-school environments. The present study is a simultaneous mixed pattern research using quantitative and qualitative data. Twenty-four science teachers participated in the study. The effect of training and applications was also evaluated in terms of gender and seniority. Data were collected through Self-Efficacy Belief Scale for Organizing Educational Trips to Out-of-school Environment and open-ended question forms. While there was no difference in self-efficacy beliefs between male and female teachers at the beginning of the project, there was a difference in favor of females at the end of the project. Also, while there was a difference in favor of senior teachers at the beginning of the project, this difference disappeared at the end of the project. As can be seen in the present study, practical activities related to out-of-school learning are very important and such training should be delivered to both in pre-service and inservice teachers.