Objectives: The purpose of this study is to determine the role of MMP-8 playing in the progress of caries. And observe the presentation of MMP-8 in caries of different degree and investigate its function in the formation of caries. Methods: we select extracted third molars which are not treated, as specimens for dental caries. According to clinical examination and radiography examination, they are divided into three groups that consists of enamel caries,superficial caries of dentin and deep caries. There are 10 cases in every group. We also select 10 cases of normal third molars teeth were also used as a control at the same time. HE stain and immunohistochemistry SABC are respectively used to examine expression of MMP-8 in different caries, while image analysis system is used for semi quantitative analysis. Results: In normal group, odontoblast arrange paleform and dentinal tubule is regular. The presentation of MMP-8 in odontoblast, predentine and fibroblast are slightlyn stained while other cells show no stain. In enamel caries group, odontoblast shows slight derangement and layers increased, pulp tissue present to network structure with inflamed cells infiltration. The expression of MMP-8 in odontoblast, predentine and fibroblast is weakly positive. In dentin superficial caries group, Dentinal tubule in caries zone is blur, fibroblast proliferate; lymphocyte, granulocyte and macrophage exist in pulp tissue. Odontoblast proliferate, predentine become thicken, MMP-8 is positive staining. In deep caries group, reparative dentin form, vacuolar Odontoblast and curved dentinal tubule under most carious lesion. fibroblast and odontoblast in pulp tissue are show positive expression, while vascular endodermis cells, surface of soften dentine and reparative dentin is strongly positive and weakly positive in dentinal matrix .Conclusion: MMP-8 participates in the caries process and development of MMP-8 inhibitors is capable of obstructing occurrence and development of caries.