Two water-soluble polysaccharides, PNW1 and PNM1, were respectively isolated from the mycelium and its culture medium of Phellinus nigricans using submerged fermentation before determining their effects on inhibiting the growth of transplantable tumors in mice. The results of the pharmaceutical experiments showed that oral administration of PNW1 and PNM1 (at a dose of 400 mg/kg) inhibited the growth of tumor of mice-transplanted Sarcoma 180 in vivo. Moreover, a higher inhibition ratio of PNW1 (74.70%) was obtained as compared with PNM1 (55.84%). The averaged molecular weight of PNW1 and PNM1 was determined to be 33 and 29 kD, respectively. Both PNW1 and PNM1 were consisted of glu- cose, galactose, mannose, arabinose and fucose. The major structural features of PNW1 and PNM1 were elucidated using partial acid hydrolysis, periodate oxidation, Smith degradation, 13C-NMR, me- thylation and GC-MS. On the basis of these results, the repeating units of PNW1 and PNM1 were estab- lished.