Polysaccharide guar gum (GG), one of the most widely used biopolymers for food applications is examined as a turbulent drag reducer, of which it reduces friction drag tremendously under a turbulent flow even though only tiny amount of guar gum is dissolved in aqueous system. Mechanical degradation of the GG is specifically being investigated in this study under a turbulent flow, in which three different molecular weights of GG were prepared using ultrasonication. Drag reduction effectiveness of the GG was measured using a rotating disk apparatus as a function of time. In order to correlate time-dependent drag reduction and mechanical degradation of GG, two different degradation models of a singlerelaxation process and a stretched-exponential model were examined, and the stretched-exponential model was found to fit the experimental data better.