Four taxa of the genus Circaea in Korea were reexamined by external morphology and investigated by anatomy, epidermal features of leaves and pollen grains with a scanning electron microscope and a light microscope. Based on the above the results, a new key was constructed, and the intraspecific relationships were discussed. Each taxon shows consistent features in external morphology. The shape of leaves was cordate in C. cordata, lanceolate in C. mollis and C. quadrisulcata, and ovate in C. alpina. The inflorescences of C. mollis were less pubescent than those of C. quadrisulcata. The nectary of flower was exserted beyond the floral tube in C. mallis and C. quadrisulcata, but that of C. cordata and C. alpina was included in the floral tube. Also, each taxon had a rhizome, but C. alpina had a tuber at the base. The anatomical characters show no significant differences among all taxa examined. Especially, C. alpina had unilocular ovary, but the remaining taxa had bilocular one. The pollen grains of all the taxa were triporate or tetraporate, and the surfaces were verrucate. In C cordata, C. mollis, and C quadrisulcate, the pollen grians had viscin threads as a typical feature in Onagraceae, but in C. alpina they did not possess them.