Three new diterpenes, 6β-hydroxyferruginol (1a), formosaninol (2a), and formosanin (2b), were isolated from the heartwood of Juniperus formosana HAY. var. concolor HAY. The structure of formosaninol was deduced to be a dimeric ferruginol with 6-O-7' and 7-O-6' linkages on the basis of spectroscopic analysis and chemical evidence. Formosanin was a dimethyl ether of formosaninol. 6α-Hydroxy-7-oxoferruginol can be converted to 6α- and 6β-hydroxyferruginols, 6-oxoferruginol and formosaninol by lithium aluminum hydride reduction and acidification. The structure of 6β-hydroxyferruginol isolated from Cryptomeria japonica was revised to 6α-hydroxyferruginol.