Abstract: Lobane-type diterpenoids are not frequently discovered from marine soft corals. In this paper, three new lobane type diterpenes, 13-methoxyloba-8,10,15(16),17(18)-tetraene ( 1 ), 8,10,13(15)Z,16E-lobatetraene ( 2 ) and 19-hydroxy-lobatetraene ( 3 ), and a new natural compound, 17,18-epoxyloba-16-acetoxy-8,10,13(15)-trien ( 4 ), co-occurring with a known related diterpenoid, 18-methoxyloba-8,10,13(15),16(17)-tetraene ( 5 ), were isolated from the South China Sea soft coral Sinularia polydactyla. The structures of new compounds were determined by extensive spectroscopic analysis and by comparison with those reported in the literature. In bioassay, all the isolates were inactive on antibacterial, PTP1B inhibitory, and immunological activities. This study increased the chemical diversity of marine diterpenoids.