Tooth impaction is one of the major and common issues in dentistry .Several factors can influence on tooth impaction .According to this, the purpose of this study was to investigate the relationship between the impaction of mandibular third molar and Guns ratio.This study is a cross - sectional survey .Among the cases referred in the last year to Oral and Maxillofacial Radiology Center in Kermanshah, 130 patients (65 patients with lower third molars was grown and 65 patients with impacted lower third molar mesioangular) aged 20 to 30 years were selected randomly. Patients who met inclusion criteria based on existing panoramic radiography, angle of inclination of the third molar to second molar (beta) angle of inclination of the third molar to mandibular base (alpha) the most distal point between the second molar to the anterior border of the ramus to width of the third molar (Guns ratio) and the angle between the posterior border of ramus and the lower edge of the mandibular (Gonial) were drawn manually. And data were collected and then the data is collected using software SPSS18 at the descriptive statistics (mean, standard deviation) and inferential statistics (t-test) and statistical analysis was conducted and the results were reported. The results showed that there were no significant differences between the angles alpha, beta, gonial and Guns ratio on the left and right in each impacted tooth and unerupted groups (p>0.05). The findings showed that there is a significant difference between the angles alpha, beta and the Guns ratio within the grown and impacted groups (p<0.05). The findings also showed that the angle alpha, beta and Guns ratio predict the impaction of the mandibular third molar at low level. However, gonial angle of mandibular third molar impaction to predict is not appropriate: According to the research findings, we can conclude that the angles alpha, beta and ratio Guns ratio can effect on third molar impaction. It also calculates the angles alpha; beta and Guns can predicted the third molar impaction.