A new tetrahydroanthracene, 3,6,9-trihydroxy-3,4-dihydroanthracen-1(2H)-one (1), six phenolics, moscatilin (2), gigantol (3), batatasin (4), moscatin (5), 9,10-dihydromoscatin (6), 10-dihydrophenanthrene-2,4,7-triol (7), and a sesquiterpenoid, corchoionoside C (8), together with two sterols $eta$-sitosterol (9) and daucosterol (10), were isolated from the stems of Dendrobium polyanthum. Compounds 1 and 2 were assessed for cytotoxic activity against two human tumor cell lines (A549 and HL-60).