Aim and Background: Treatment success is one of the main concerns of orthodontists. Treatment outcomes may be assessed using various indices like the PAR index. The present study was conducted to evaluate the treatment success rate and the related factors in the Department of Orthodontics of the Dentistry School of Islamic Azad University based on the PAR index from 2009 to 2014. Method and Materials: The present cross-sectional study was among 100 eligible patients. The pre- and post-treatment casts were assessed by evaluating the archives through contact point displacement, buccal occlusion (anterior, posterior, transverse and vertical planes), vertical anomalies (open bite and deep bite), transverse anomalies (cross bite), anterior-posterior anomalies (over jet) and midline anomalies. Their pretreatment casts were presented, examined and recorded along with the patients’ time to the orthodontics department. The type of malocclusion, gender, and age were also rated based on the PAR index. Results: The present study was conducted on 200 pre-treatment and post-treatment study models obtained from 100 treated patients (32% males and 58% females). The participants had a mean age of 17.6±4.29 ranging from a minimum of 10.4 to a maximum of 33.6, among whom, 36% had extracted teeth and 54% did not. The mean duration of treatment was 2.3 years ± 0.6 months, with a minimum duration of 16 and a maximum of 48 months. 66% of the participants were suffering from CL I, 24% from CL II, and finally 10% from CL III malocclusion. The distribution of the patients’ success and failure showed an 85% complete success (more than 70%), 13% relative success (30% to 70%) and 2% failure (less than 30%). Conclusion: The mean difference and reduction in the weighted PAR score was calculated as 21.3; while the mean reduction was 89.98%, revealing significant progress in the orthodontic treatments. A total of 85% of the cases were completely successful (more than 70%), 13% were relatively successful (30% to 70%) and 2% were unsuccessful (less than 30%), suggesting a standard treatment with very high quality. No significant difference was observed between the treatment success rate and the variables including age, gender, duration of treatment, having or not having extracted teeth and type of malocclusion.