The dipole-dipole interactions between polar nitrile groups of polyacrylonitrile (PAN) affected the molecular weight distribution (MWD)-dependent viscosity behavior of the solutions in N,N-dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO). In both solvents, the 16 wt% PAN solutions of broader MWD but same weight average molecular weight (MWw) of 150,000 showed more prominent shear-thinning behavior, agreeing qualitatively with the theoretical prediction by Middleman. The dependence of the thinning behavior on the MWD was more pronounced with DMSO. The decrease of the power-law index by broadening of the MWD was more noticeable in DMSO than in DMF. The UV/vis and FTIR spectra confirmed that DMF produced stronger intra- and intermolecular dipole-dipole pairs between nitrile groups of PAN than DMSO. This indicated that physical association between polymer molecules limited the internal plasticizing effect of the low MWw molecules by disturbing the chain interdiffusion and shear-induced disentanglement. Over the whole range of frequency measured, the higher content of the higher MWw molecules in the broader MWD solutions resulted in the increase of dynamic viscosity and relaxation time but decrease of loss tangent.