The diffusion coefficients of four polymer-solvent mixtures at 34.0℃ were measured using a Mach-Zehnder diffusiometer. Monodisperse(M_w/M_n<1.05) polytetrahydrofuran (PTHF) with and without OH-end group were chosen for this work to study the effects of association of polymer molecules on the diffusion coefficient. In order to minimize the interaction between solvent and polymer molecules, bromobenzene having no hydrogen bond-forming capability was employed as a solvent. The experimental results showed that the OH end group indeed affected diffusion property of dilute polymer solution. An association model was developed in this work and the experimental results were interpreted based on the model.