On high performance liquid chromatography, the caffeoylquinic acid (CQ) occupying the highest proportion of the water-ethanol (7:3) extract of Aster glehni (Compositae) leaves was 3-O-p-coumaroylquinic acid ($6.10{pm}4.22$ mg/g of dried weight) among CQs tested. The $IC_{50}$ of the water-ethanol (7:3) extract was $4.23{pm}0.24{mu}g/mL$ in the peroxynitrite ($ONOO^-$)-scavenging assay. Phytochemical isolation from A. glehni extract yielded three kaempferol glycosides. The water-ethanol (7:3) extract and both p-coumaric acid and caffeic acid, phenylpropanoid moieties of CQs, had sedative effects in pentobarbital-induced sleeping time in mice and anticonvulsant effects in pentylenetetrazole-induced mice. Furthermore, the phenolic substance-rich W-E (7:3) extract of A. glehni could be used to treat anxiety or convulsion partly due to its peroxynitrite-scavenging mechanism.