The curricular integration of the basic sciences and clinical medicine has been conducted for over 40 years and proved to increase medical students' study interests and clinical reasoning. However, there is still no solid data suggesting what time, freshmen or year 3, is optimal to begin with the integrated curriculum. In this study, the integrated courses on cardiovascular and respiratory systems were performed to part of year 1 and year 3 medical students while non-participant students acted as control. We tried to explore the optimal time through comparison of the exam results and questionnaire of participated students. It was demonstrated that year 3 participant students got better exam score than year 1 students did, and the questionnaire showed that it might be due to the year 1 participants difficultly caught up with the contents of integrated courses without appropriate background knowledge. Three years later, the participant students got higher ability to analytical thinking of clinical diseases in comparison to non-participant students, while it did not improve the acquirement of their clinical practical skills. Taken together, our study in Jinan University School of Medicine indicated that the integrated courses would be approximately effective if combined to conventional medical teaching at year 3 after the students obtain relevant basic sciences knowledge.