Chemical investigation of the cytotoxic fraction of Koelreuteria henryi resulted in the isolation of three cyclolignans. A new cyclolignan, named koelreuterin-1 was elucidated as furo[3',4':6,7]naphtho[2,3-d]-1,3-dioxol-6(8H)-one,5-(7-methoxy-1, 3- benzodioxol-5-yl)[1]. Two known cyclolignans were characterized as austrobailignan-1 [2] and austrobailignan-2 [3]. The structure elucidation of 1 was based on extensive 1H- and 13C-nmr spectral analyses. Further chemical conversion of 2 to 3 and oxidative transformation of 2 to 1 unambiguously confirmed the structure of 1. The cytotoxicity and tubulin polymerization inhibitory activity of 1-3 are discussed.