The aim of this study is to determine the competencies of preservice science teachers in designing experiments based on Predict-Observe-Explain (POE) method. For this purpose, the holistic single case design, which is one of the qualitative research methods, was used in the study. The research was carried out with the participation of 57 science pre-service teachers who were taking Science Teaching Laboratory Applications-2 course in a university in Istanbul in the 2018-2019 academic year. Easily accessible situation sampling was used in the selection of the study group. Within the scope of the study, a four-hour seminar lesson on POE method was given to preservice teachers. Three weeks after the seminar, pre-service teachers formed groups and designed an experiment based on the POE method on fermentation and made a report on this design. In the study, as a data collection tool, the 11-item POE design assessment tool (POED), developed by the researchers as a result of the literature review, was used in order to evaluate the experimental designs based on the POE. As a result of the research, it is seen that the teacher candidates are sufficient in the observation stage (72%), partially sufficient in the prediction stage (45%) and insufficient in the explanation stage (20.5%). When all stages of experimental designs based on POE were evaluated, it was determined that the percentage of success was 49.5% and this value was close to the average value and that the pre-service teachers were partially sufficient in their designs. In this direction, it is recommended to use innovative methods such as argument formation, questioning, critical thinking more frequently in teacher training programs in order to improve the experimental designs based on POE of pre-service teachers.