Three new kavalactone dimers, designated as yangonindimers AC (13), along with one known analogue were isolated from the roots of Piper methysticum. Their structures were elucidated via extensive analysis of their 1D, 2D NMR and mass spectroscopic data. All these dimers possess a skeleton featuring a cyclobutane ring connecting two kavalactone units. Compounds 1–4 were evaluated for their cytotoxic activities against human tumour cell lines NCI-H46, SW480 and HepG2, but none showed significant activity.