The purpose of this study was to compare the effectiveness and efficiency of two teaching models, the expository presentation of Merrill and Tennyson's model and Gagne's model, in,acquisition and maintenance of square and triangle concepts in students with mental retardation. The participants of the Study were chosen from students with mental retardation who attended a private special education center located in Kahramanmaras. Three buys and one girl participated in the research. The design of the study was an adapted alternating treatment design. Two criterion referenced tests were developed and administrated to assess the discrimination levels of square and triangle in participating students. The results of the study showed that the Gagne model was more effective than the Merrill and Tennyson model on the concept acquisition and maintenance in two of the participants. However, the effectiveness of two models did not differ significantly in the remaining two participants. Both models were equally effective with these students. The results also showed that the Gagne model required less teaching time with three participating students compared to the Merrill and Tennyson model. Results of the study were discussed and recommendations for further research were provided.