Aim of this study was to evaulate performance tasks related to Genetics and Biotechnology subject in view of peer groups and a science teacher through many-facet Rasch model (MFRM). Survey method was used. Students prepared these posters during the autumn semester of the 2013-2014 school years. Rasch model's surfaces are respectively: 10 peer groups and one science teacher, criterias and performance tasks which were handmade posters related to genetics and biotechnology. Each group has prepared one material. A form which was revised by the teacher was used as data collection tool. FACETS program was used for data analysis. Findings reveal that Group 74 is the most lenient group, teacher was the most severe assessor; P4 was found to be the most successful and P6 was found most unsuccessful; Knowledge-accuracy criteria was reached easily and reference criteria was reached hard. In conclusion, it is thought that the results of this study demonstrated MFRM can be used in elementary science education effectively to handle polytomous data in performance and peer assessment.