Background and objective: Musculoskeletal disorders in dentists due to abnormal postures may result in economic and health problems. To prevent these problems, the working posture of dental students should be assessed and corrected in dental procedures. This study was undertaken to assess the effect of instruction regarding ergonomic principles on them by using the Rapid Entire Body Assessment (REBA) method. Material and methods: The posture of 117 students was assessed by REBA before and after of the "Ergonomic principles in dentistry" course. The sample size was selected based on previous studies on this issue. REBA scores were assessed by SPSS software using Friedman-test. Results: The statistical analysis of data gathered from this study showed that the ergonomic structure had a significant effect only in the group of right-handed females (P=0.046). In other groups, no significant change was observed (P>0.05). There was no significant change before and after instruction in REBA classifications (negligible, low risk, medium risk, high risk, very high risk) (P>0.05). There was no relation between the types of positions and REBA score (P>0.05). Conclusion: Ergonomic instruction can significantly decrease musculoskeletal disorders in female dentists who use their right-hand to work. In the other groups, however, the absolute amount of danger became better, but this change was not statistically significant.