Plaunotol (1), a known antiulcer drug, is the most important component of the Thai folk medicinal plant, Plau-noi, which has remarkable antipeptic ulcer activity. Recently, it was found that plaunotol (1) has antibacterial activity against Helicobacter pylori, a causative agent in gastric ulcers and gastric adenocarcinoma, for example. In our investigation of the practical synthesis of 1, we have developed an efficient method for stereoselective synthesis of trisubstituted olefins via a Z-selective Wittig reaction. The olefination of readily available aliphatic α-acetal ketones with triphenylphosphonium salts in the presence of a potassium base and 18-crown-6 ether proceeded with excellent Z-selectivity. The Z-selective olefination provides a useful method for the construction of a range of trisubstituted olefin moieties; the practical and stereoselective total synthesis of plaunotol (1) was achieved via this Wittig reaction.