This article discusses assessment practices in primary education and implicit theories in teaching. Cultural practice and social interactions in the classroom create personal experiences, which are the basis for teachers’ individual mental representations, known as implicit theories. These conceptions guide their teaching, but they are not generally studied, unlike the theories of the scientific community. Although implicit theories may be different, teachers from the same educational institution generally share them and they regulate them in relation to the context. This study seeks to identify and interpret the implicit theories on learning assessment of a primary school teacher in Puno, Peru. The methodology used was grounded theory. For this purpose, the empirical method, such as participant observation and in-depth interviews, was used. The main result was identifying in the teacher an assessment practice with a prevalence of constructive and interpretive theory traits.