Two novel tricyclic spirolactones bearing long linear alkyl chains, yaoshanenolides A (1) and B (2), formed by Diels-Alder[4 + 2] cycloaddition of a molecule of each butenolide with β-phellandrene, were isolated from the bark of Machilus yaoshansis. Their structures and absolute configurations were determined by extensive spectroscopic methods, especially 2D NMR and ECD data analysis. The proposed biosynthetic pathway is discussed. Both compounds exhibited nonselective cytotoxic activities against several human cancer cell lines.