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