This research entitled the application of written feedback  assessment to improve the mastery of student concepts. This study aims to determine the mastery of student concepts through the application of written feedback  assessment in the learning of ecosystem materials. The research method used in this research is descriptive method. The sample in this study is the 30 students of SMP Negeri 6 Sukabumi class VII G. Sampling of this research was taken randomly by using Purposive Sampling technique. The instrument used in the research are Student Worksheet which is used to measure the mastery of the concept of the student and the questionnaire used to obtain additional data in the form of student responses to the learning with the application of written feedback  assessment. The results showed that the provision of written feedback  assessment in learning can improve the mastery of student concepts. Applying written feedback  assessments can also encourage students to be more active in their learning process, building student motivation to be willing to process and improve their abilities. In other words the application of written feedback  assessment is very feasible to apply in school learning.