Objectives : By investigating antimicrobial activity of natural extracts, identifying its usefulness as antibiotic material to oral bacteria. Methods : Antimicrobial activity tests of 25 natural extracts were implemented on 6 type strains alleged to cause dental caries and 10 clinical strains isolated and identified from dental caries. Results : Among medicinal plants, Coptis japonica, Scutellaria baicalensis, Dictamnus dasycarpus showed antimicrobial activity. Among lichens, 3 methanol extracts and 6 acetone extracts showed antimicrobial activity. Minimum Inhibitory Concentration(MIC) test of Usnea aurantiacoatra KoLRI 004184, an acetone extract of lichen with the highest antimicrobial activity, on 6 type strains involved in dental caries and 10 strains isolated from dental caries resulted as follows; Actinomyces oris 1041 12㎍/ml, for Corynebacterium durum 3151 13㎍/ml, for Rothia dentocariosa KCTC 3204T 14㎍/ml, for R. dentocariosa 911 15㎍/ml, for R. dentocariosa 1822 12㎍/ml, for Lactobacillus casei KACC 12413T12㎍/ml, for L. acidophilus KACC 12419T 16㎍/ml, for L. rhamnosus 2421 15㎍/ml, for Streptococcus mutans KCTC 3065T 15 ㎍/ml, for S. mutans 121 13㎍/ml, for Streptococcus oralis 2221 18㎍/ml, for S. salivarius KCTC 5512 14㎍/ml, for S. salivarius 122 12㎍/ml, for S. sanguinis KCTC 3284T 14㎍/ml, for S. sanguinis 912 14㎍/ml, for Neisseria sp. KEM232 12㎍/ml. Conclusions : Even a small amount of extract from lichen Usnea aurantiacoatra KoLRI 004184 showed very high level of antimicrobial activation against all 16 bacterial strains involved in dental caries.(J Korean Soc Dent Hygiene 2012;12(1):81-91)