The purpose of the study was to examine the effect of activity-based nanoscience and nanotechnology education (ABNNE) on pre-service science teachers' (PST') conceptual understanding of nanoscience and nanotechnology. Within this context, the study was conducted according to mixed methods research with the use of both quantitative and qualitative methods. The participants were 32 PST who were determined by using criterion sampling that is one of the purposive sampling methods. ABNNE was carried out during 7 weeks as 2 h per week in special issues at physics course. Design and implementation of ABNNE were based on "Big Ideas" which was found in literature and provided guidance for teaching nanoscience and nanotechnology. All activities implemented during ABNNE were selected from literature. "Nanoscience and Nanotechnology Concept Test (NN-CT)" and "Activity-Based Nanoscience and Nanotechnology Education Assessment Form (ABNNE-AF)" were used as data collection tools in research. Findings obtained with data collection tools were discussed with coverage of literature. The findings revealed that PST conceptual understanding developed following ABNNE. Various suggestions for increasing PST conceptual understanding of nanoscience and nanotechnology were presented according to the results of the study.