The rattans of <i>Spatholobus suberectus</i> Dunn are a traditional Chinese medicine activating blood circulation and removing stasis. They have often been used for the traditional Chinese medicinal treatment of breast cancer in modern China. In this study, four novel isoflavanes (<b>1</b>-<b>3</b> and <b>5</b>) and four known analogues (<b>4</b> and <b>6</b>-<b>8</b>) were isolated from an ethanolic extract of the rattans of <i>S. suberectus</i>. Their structures were elucidated by extensive spectroscopic analyses and electronic circular dichroism studies. MCF-7 and MDA-MB-231 human breast cancer cell lines were used to evaluate the cytotoxic effects of the isolates. Interestingly, compounds <b>1</b> and <b>2</b> only inhibited the proliferation of MCF-7 cells, while compound <b>6</b> showed a selective cytotoxicity against MDA-MB-231 cells. However, compound <b>4</b> had significant cytotoxicity against both MCF-7 and MDA-MB-231 cell lines.