In this paper, we analyse the internal and external factors that cause learning difficulties for students with mental retardation. The research subjects were 7 mentally retarded students, to carry out this study the observation sheet was designed to observe students while at school, questionnaire for students and teachers, and interview sheets for students, teachers and parents. The results obtained revealed that internal factors such as motivation and interest had no effect as well as for readiness and attention, while the extrenal factor, which is the school and family environment, influences. So it can be concluded that what affects the learning biology difficulties of retarded students is external factors.