This research aims to describe the implementation of contextual learning mathematics for children with light mentally disabled at the VII grade SLB Negeri Purworejo. This research includes (1) organizing mathematics learning materials, (2) mathematics learning strategies, (3) obstacles experienced during the implementation of learning, (4) student responses during the implementation of learning. This research uses descriptive qualitative research method case study type. The Subjects of the research is light mentally disabled children at the VII grade. The data collection uses observation, field notes, interviews, and documents. Data analysis techniques consist of reducing data, presenting data, and concluding. Verification of data validity using technique and source triangulation. The result of this research concludes that the implementation of mathematics learning for student mentally disabled consist of: (1) Most of the organizational aspects of mathematics learning materials for children with mentally disabled related to individual programs have not been implemented, only some aspects are implemented that are students of mentally disabled participate in the implementation learning and materials provided are beneficial for everyday life; (2) All aspects of the mathematics learning strategy for children with mentally disabled including reinforcement, punishment, and materials classified according to child development have not been implemented; (3) obstacles experienced by teacher during the learning includes: the number of children with special needs that exist in one class with various capabilities, so that the material has been given is still general (4) teacher has lack of ability in presenting the model as a more realistic learning example (5 )the student mentally disabled student responses during learning are positive.