In this article we describe the use of mental states approach, more specifically the belief-desire-intention (BDI) model, to implement the process of affective diagnosis in an educational environment. We use the psychological OCC model, which is based on the cognitive theory of emotions and is possible to be implemented computationally, in order to infer the learner's emotions from his actions in the system interface. In our work we profit from the reasoning capacity of the BDI model in order to infer the student's appraisal (a cognitive evaluation of a person that elicits an emotion), which allows us to deduce student's emotions. The system reasons about an emotion-generating situation and tries to infer the user's emotion by using the OCC model. Besides, the BDI model is very adequate to infer and also model students affective states since the emotions have a dynamic nature.