Background: Based on the Haddon matrix, several factors such as environment, host, and agent can affect the severity of traffic-related injuries. Objectives: This study aimed to determine the factors affecting the severity of injuries due to traffic accidents using the Haddon matrix. Methods: In this cross-sectional study, all traffic accident victims (n=2015) referred to Imam Khomeini Hospital in Urmia during 2015-2016 were selected by the census method. Demographic data of the patients and data regarding Haddon risk factors was collected and then analyzed by descriptive statistics, chi-square test, and multivariate logistic regression using the SPSS 18 software. Results: The mean age of the participants was 33.63±18.53 years. Most of the victims of traffic accidents were male (age range, 17-30 years) with severe and critical injuries. The most common mechanisms of injury were car-pedestrian collisions (27.9%), car overturning (31.1%), and collisions between two cars (26.3%). Violation of speed limit (73.2%) and violation of right-of-way (17.9%) were the most common moving violations. Additionally, the results of the multivariate analysis regarding vehicle safety devices showed that there was a significant relationship between the time and location of the accident and the severity of the injury (p <0.001). Conclusion: The findings of this study reveal that many factors, including using vehicle safety tools and time and location of the accidents have been implicated in the severity of accident-related injuries. Hence, appropriate planning and training and national and international measures can be helpful in reducing injury severity.