Abstract: The stability constants of Co(Ⅱ) complexes with weathered coal fulvic acid are determined by both Schubert method and Stevenson method at pH=5.3±0.2, I =0.001～1.0 mol/kg and (30±1)℃. The modified Schubert method is also used to estimate the stability constants of Co(Ⅱ) complexes with the fulvic acid. It is shown that the trend of forming polynuclei complexes of Co(Ⅱ) fulvic acid increases with increasing ionic strength in the solution and that 1∶1 and 2∶1 type complexes, respectively, are formed at different ionic strength. A comparison between the values of lg β obtained by different methods is made.