Artemisia capillaris and Artemisia iwayomogi are weeds used as medicinal plants in Korea under the name "Injin". I collected leaves of A. capillaris and A. iwayomogi, examined them for phytotoxic effects from volatile substances and determined the composition of monoterpenes in the leaves. The effects of volatile substances from each species on seed germination and radicle elongation in each of the two Artemisia species were assessed. The volatile substances of A. capillaris did not negatively affect the seed germination of A. capillaris, but they did inhibit radicle elongation. Rates of seed germination of A. iwayomogi decreased when the seeds were exposed to high concentration of A. capillaris volatile substances. The inhibition of seed germination and radicle elongation by volatile substances from both Artemisia species was stronger for A. iwayomogi than for A. capillaris. I identified the monoterpenoids from the leaves with a gas chromatograph-mass spectrometer (GC-MS). The main constituents of A. capillaris were acenaphthylene (37.91%), $eta$-pinene (12.08%), 4-carene (10.61%) and $gamma$-curcumene (9.92%), while those of A. iwayomogi were germacrene-$delta$ (32.15%), borneol (21.24%), camphor (20.45%) and trans-caryophyllene (7.75%).