Volatile terpenes responsible for allelopathic activity in four weed species were identified using gas chromatography (GC) and GC-mass spectrometry. Artemisia asiatica Nakai contained 26 volatile terpenes, consisting of 16 monoterpenes and 10 sesquiterpenes, whereas there were only four terpenes in Trifolium repens L. and three terpenes each in Portulaca oleracea L. and Capsella bursa-pastoris (L,) Medik. Suspected allelochemics were ${alpha}$-phellandrene, 1,8-cineole, limonene, ${alpha}$-pinene, borneol, selinene, and caryophyllene in A. asiatica, ${alpha}$-pinene and ${eta}$-caryophyllene in T. repens, and ${alpha}$-pinene in C. bursapastoris. No these compounds were found in P. oleracea.