Teachers' understanding of formative assessment is quite essential for its implementation in everyday classroom practices [Heritage, M. (2007). Formative assessment: What do teachers need to know and do? "Phi Delta Kappan," 89(2), 140-145]. This study investigates the development of teachers' perspective on formative peer assessment when it is newly introduced into their teaching. A group of 18 science teachers participated in design-based research project in which they developed and enacted inquiry-based teaching units with embedded formative peer assessment instances. Teachers' perspective on formative peer assessment was captured by means of group discussions and reporting tool at different stages of the project (before teaching the experimental units, after having taught the units and 20 months after that). Data were analysed by thematic analysis. The findings show that immersing teachers into this relatively new method, formative peer assessment, especially when they are co-investigating their own practice (in research project), may have significant impact on their conceptualisation and implementation of formative assessment in general.