Phainanoids A-F (1-6), six highly modified triterpenoids with a new carbon skeleton by incorporating two unique motifs of a 4,5- and a 5,5-spirocyclic systems, were isolated from Phyllanthus hainanensis. Their structures with absolute configurations were determined by spectroscopic data, chemical methods, and X-ray crystallography. Compounds 1-6 exhibited exceptionally potent immunosuppressive activities in vitro against the proliferation of T and B lymphocytes. The most potent one, phainanoid F (6), showed activities against the proliferation of T cells with IC50 value of 2.04 +/- 0.01 nM (positive control CsA = 14.21 +/- 0.01 nM) and B cells with IC50 value of <1.60 +/- 0.01 nM (CsA = 352.87 +/- 0.01 nM), which is about 7 and 221 times as active as CsA, respectively. The structure-activity relationships of 1-6 are discussed.