Engineering classrooms often contain mixed-ability students with less interest in language study. These students come from different backgrounds and different mediums of instruction. Grammar translation method, communicative approach, or multimedia fail to entertain one or the other group of students in the same classroom. Diagnostic Feedback Model can be utilised for effective language teaching in such mixed-ability classrooms. With a descriptive research design, an evaluative study is conducted in VV College of Engineering, Tisaiyanvilai. 200 students and 10 teachers are involved in the study. Meetings are conducted periodically. Tasks for the pre-tests and criteria for evaluation are designed by the teachers. The pre-assessment contains items to check listening, body language, fluency, and accuracy of the students in speaking. The criteria provides 5 – 1 range of marks for each sub-skill in speaking. The students are made to speak and are evaluated by the teachers using the criteria. Based on the diagnostic feedback model, data consolidation is done by the teachers. The diagnostic feedback model provide the teachers with the strengths and areas of improvement of the students. According to the overall scores, the students are classified into Beginner / Intermediate / Proficient instead of Below Average / Average / Above Average. Activities for each group are defined uniquely, and training is conducted separately for each group. At the end of the course, post-assessments are conducted using the same criteria. In the post-assessments, the average scores of 'Beginners' have increased from 20 to 35, the average scores of 'Intermediate' students have increased from 30 to 43, and the average scores of 'Proficient' students have increased from 40 to 48. This implies that diagnostic feedback model works well in mixed ability classrooms in engineering colleges.