Abstract: Objective　To study the relationship between impaction types and extraction time and accordingly make a new classification of mandibular third molars. Methods　There were 413 mandibular third molars included in the study. The impaction types were classified according to Pell-Gregory classifications (ramus and occlusal) and Winter classification. Ri、Oi、Wi were used to represet the index of immpaction respectively. Ci represents comprehensive impaction index of mandibular third molars. Ci=Ri+Oi+Wi. Extraction difficulty of mandibular third molars was measured by extraction time. ANOVA was used to compare the difference of extraction time between every two comprehensive impaction indexes of mandibular third molars. Classify mandibular third molars according to comprehensive impaction index and extraction time. Results　The extracion time became longer with greater comprehensive impaction index. There were significant differences in extraction time among mandibular third molars in some comprehensive impaction indexes. Mandibular third molars could be divided into six categories(Ⅰ～Ⅵ) according to comprehensive impaction index and extraction time. Conclusion　There was a close relationship between the extraction difficulty and impaction types of mandibular third molars. The new established classification of mandibular third molars can reflect their extraction difficulty better.