The molecular weight of Utea Modified Sulfonated Phenol Formadehyde Resin (spu) was determined in its aqueous solution by dynamic membrane osmometry．Considering the proper-ties of water．the raising line of H-dV/dt．which was drawn out near the estimated equilibrium point，was chosen as a standard for obtaining the molecular weight of the sample．The second virial coefficients A2 of SPU found by this method were larger than that of the statistical coil．Z3／C2’calculated from A2 is a parameter characterizing the effective charge of the polyion．The value of Z2／C2’were consistent with the depressions of the water-]oss of the SPU-treated drilling mud，i.e.，the greater the Z2／C2’the better the depression is．The value of Z2/C2'may have certain meaning.