Background and Aim. Some complications, including infection, may occur following the surgeries of impacted teeth. In order to reduce the risk of infection, it is necessary to evaluate the risk factors of infection after third molar surgeries. Although research has been published in this area, the risk factors and causes of infection are still unclear. This study aimed to investigate infection incidence and effective factors in hard tissue impacted mandibular third molar surgeries in the Iranian population. Materials and Methods. This cross-sectional study was performed on 148 patients. The incidence of infection was recorded one week, two weeks, and one month after surgery. Independent variables included age, gender, the presence of periapical radiolucent lesions measuring & GE;3 mm, the caries of the adjacent second molar, education level of the surgeon, difficulty index, suture type, and flap type. The outcome variable was the incidence of infection. Results. Among the study subjects, 37.2% were male and 62.8% were female. The mean age was 25.24 +/- 4.31 years at the time of surgery. Although early onset infection occurred in 3.4% of patients, none of them developed a delayed-onset infection during the four weeks after surgery. Pederson difficulty index (OR = 3.81, P=0.03) and the education level of the surgeon (OR = 0.14, P=0.021) were associated with postop infection. Conclusions. Based on the results of the current study, postop infection was rare. Furthermore, both the Pederson difficulty index and the education level of the surgeon could predict the risk of infection after impacted mandibular third molar surgeries.