In this study, it was aimed to investigate the mathematical modelling activity preparation processes of secondary school mathematics teachers who participated in mathematical modelling training. The study was conducted with six mathematics teachers working in secondary schools. In the study designed as action research, two action plans were applied to the teachers. The first activity forms collected from the teachers before the application, the second activity forms collected as a result of the theoretical training about modelling and the activity forms collected at the end of the active participation of the teachers in the modelling activities constitute the data source of this study. An activity evaluation form was used in data analysis. The results of the study showed that teachers initially had deficiencies in preparing modelling activities in accordance with all criteria. After the theoretical training given, it was determined that teachers’ competencies in preparing modelling activities improved very little. In addition to theoretical training, it was seen that the learning environment prepared to ensure teachers’ active participation in modelling activities affected their activity preparation competencies more positively. The results show that teachers’ theoretical knowledge deficiencies should be eliminated in modelling teaching and learning environments should be prepared for their active participation in mathematical modelling activities.