We studied Karen ethnomedicine in Wa Do Kro village,Tha Song Yang District, Tak province in Thailand. Our aims were to document the medicinal uses of local plants and to evaluate their importance to the villagers. We also examined the relative importance of each ethnomedicinal species for treating any ailments. We interviewed 38 informants and found 177 medicinal species, belonging to 76 families and we recorded 3,168 use reports.The most used plant families were Fabaceae (18 species) and Asteraceae (9 species). The most used plant parts were leaves (25%),roots (22%), and whole plants(18%). To compare the usefulness of each medicinal species we calculated the ethnobotanical indices relative frequency of citation (RFC) and cultural importance index (CI). The two use categories with most reports were infection/infestation and muscular-skeletal system disorders. Our result showed that medicinal plants still played an important role in the daily life of Karen people as evidenced by the high number of medicinal species and use reports.Among these medicinal plants, the most important species with the highest CI values were Blumea balsamifera (L.)DC. and Chromolaena odorata (L.) R. M. King & H. Rob. and Acorus calamus L. and Sambucus javanica Blume which had the two highest RFC values.