The methanol extract of Bletilla striata, an ornamental orchid in eastern Asia, exhibited plant growth-inhibitory activity. It was purified by continuous chromatography, based on the inhibitory activity against the growth of lettuce seedlings, resulting in two glycosidic compounds, militarine and dactylorhin A. The $EC_{50}$ values of militarine and dactylorhin A against the radicle elongation of the lettuce seedlings were 0.28 and 0.88 $mmol;L^{-1}$, respectively. The amount of militarine and dactylorhin A in the methanol extract of the aerial part of B. striata was calculated to be 5.6 and 7.5 $mg;g^{-1}$ fresh weight, respectively. The inhibitory activity of militarine and its content in the methanol extract revealed that the plant growth-inhibitory activity of the extract of B. striata was mainly related to militarine. The inhibitory activity of militarine against the growth of Italian ryegrass and timothy were of the same level as that of lettuce.