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 $12{mu}g/ml$, for Corynebacterium durum 3151 $13{mu}g/ml$, for Rothia dentocariosa KCTC $3204^T$ $14{mu}g/ml$, for R. dentocariosa 911 $15{mu}g/ml$, for R. dentocariosa 1822 $12{mu}g/ml$, for Lacto-bacillus casei KACC $12413^T$ $12{mu}g/ml$, for L. acidophilus KACC $12419^T$ $16{mu}g/ml$, for L. rhamnosus 2421 $15{mu}g/ml$, for Streptococcus mutans KCTC $3065^T$ $15{mu}g/ml$, for S. mutans 121 $13{mu}g/ml$, for Streptococcus oralis 2221 $18{mu}g/ml$, for S. salivarius KCTC 5512 $14{mu}g/ml$, for S. salivarius 122 $12{mu}g/ml$, for S. sanguinis KCTC $3284^T$ $14{mu}g/ml$, for S. sanguinis 912 $14{mu}g/ml$, for Neisseria sp. KEM232 $12{mu}g/ml$. 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.