The molecular weight characterization of partially hydrolyzed polyacrylamide (HPAM) for enhanced oil recovery use is rather difficult because of its ultrahigh molecular weight copolymer and polyelectrolyte behaviors in solution. In this work the effects of aqueous NaCl solution concentration and degree of hydrolysis of polymer on molecular dimension were studied. A simple and precise method for determining molecular weight of HPAM is presented. The molecular weight of HPAM with any degree of hydrolysis can be calculated from the [eta] - M(w) equation of unhydrolyzed PAM in an H2O system by measuring [eta](Cs-->infinity), of HPAM obtained in aqueous NaCl solutions by extrapolating salt concentration to infinity. Because the values of [eta](Cs-->infinity) Of HPAM of different degrees of hydrolysis are all equal to the corresponding [eta] value of the unhydrolyzed PAM of the same degree of polymerization, the molecular weight of HPAM of any degree of hydrolysis can thus be calculated from the [eta] - M(w) equation for PAM homopolymer.