Peer assessment (PA) is a process that involves students assessing each other’s work and providing their peers with (elaborated) feedback. This thesis investigates how an elaborated form of formative PA can be implemented in the mathematics class to promote students’ achievement in mathematics. A mixed methods investigation at a third-grade mathematics class was conducted. For this investigation, a teacher’s guide and a student guideline were developed. The teacher’s guide helps the mathematics teacher in implementing PA in the class. The students’ guideline was developed to guide students doing online PA during distance learning due to the COVID-19 pandemic. Using this student guideline can require more effort for the teacher to monitor students’ participation and more effort for students to do elaborated online PA. Students were trained with the technique “two stars and a wish” for providing peer feedback. With this feedback technique, students could provide their peers with two reinforcing feedback and one suggestive feedback. The feedback has to first be written in their peer’s work and later be elaborated when the work is being returned. As a result of this investigation, implementing PA promoted students to interact and learn from one another during online distance learning. Evidence was found that by implementing PA, students were more involved in their learning process, and students’ achievement increased. PA can be used as an effective learning method for students. However, it is preferably for PA to be implemented during physical class, especially when it is implemented the first time.