Abstract: The rheology of the cationic guar (CG) solution was measured and the effects of potassium oleate (KOA) upon the rheological properties of CG solution were studied. The steady shear viscosity measurement has shown that the viscosity of CG solution increased dramatically in the presence of KOA. The viscosity enhancement of KOA upon CG solution can be approximate three orders in magnitude. The gel-like formation of CG solution is observed at the high concentration of KOA. The excess addition of KOA results in the phase separation of CG solution. The oscillatory rheological measurement has shown that the crossover modulus Gc (corresponding to either storage modulus G′ or loss modulus G″ at the frequency w_c where G′ equals G″) for CG solution, decreases with the increasing the concentration of KOA in solution. On the other hand, the apparent relaxation time τ_app (=1/w_c) increases with increasing the concentration of KOA in solution. Our experimental results suggest that for surfactant such as KOA which has a stronger tendency to form micelles in solution, the cooperative hydrophobic interaction of polymer bound to surfactants is less necessary to the formation of aggregates in solution, especially at the high concentration of surfactants. In fact, with the increase of the concentration of KOA, the number of the aggregates which associate polymer together decreases whereas the intensity of these aggregates increases. The effect of temperature upon the aggregation is also significant. With the increase of temperature, the number of the aggregates increases whereas the intensity of these aggregates decreases, probably because the ionization of KOA increases at high temperature.