Background: Evaluating teaching is becoming more important because it is more frequently used for making administrative decisions about teachers. Purpose: To explore the possible effects of student and class characteristics on students' evaluation of classroom teaching. Method: Three factors--student academic level, class size, and subject matter--were analyzed with the data of students' ratings of classroom teaching in 12medical schools in China. Student academic level ranged from 1st year through 4th year. Class sizes (range = 30 to 200 students) were divided into 4 groups. Four general subject matter categories--basic sciences, basic medical sciences, clinical sciences, and preventive medicine--were examined. Results: Analyses showed that teachers who taught smaller classes and who taught in clinical sciences received higher ratings from students. Conclusions: When students' ratings are used by administration for evaluating classroom teaching, attention should be paid to the variables of class size, year, and subject being taught.