So far the learning assessment developed in elementary schools is only limited to the assessment of learning outcomes. The test instrument used only measures cognitive abilities and results in learning outcomes scores that cannot be used to measure students' abilities comprehensively. Assessment for learning is a learning assessment paradigm that can produce a comprehensive picture of students' abilities and can be used to improve the learning process on an ongoing basis. This study aims to describe the form of assessment for learning on Pirie & Kieren's understanding models in mathematics learning. This research is qualitative research using a case study approach. Data in this study were collected through direct observation of critical events, document inspection of written test results, and semi-structured interviews of research subjects. The data is analyzed qualitatively including category collection, direct interpretation, pattern formation, and naturalistic generalization. The credibility of the data in this study was verified using the data triangulation method. The results showed that the research subjects were in the formalizing layer in the learning of quadrilateral. In the image having layer research subjects experienced obstacles and folding back to the image making layer to reconstruct her understanding. Unfortunately, the formalizing layer of research subjects again experienced obstacles that resulted in it not being able to grow to the next layer. Through the use of the assessment approach for learning in the Pirie-Kieren understanding model, it can be seen that students' abilities comprehensively and the obstacles experienced during the learning process take place.