Imidazolium-based ionic liquids have been attracted enormous attention in making biofuels from biomass. Due to the high viscosity, the organic solvent in general is applied to improve the properties of the ionic liquids. In this work, the promising substances of N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and pyridine (PYR) are selected to investigate for ... [Show full abstract] the ionic liquid of 1-butyl-3-methylimidazolium chloride. The kinematic viscosity is measured by an Ubbelohde capillary viscometer at atmospheric pressure (0.0967 MPa) from 303.15 K to 353.15 K. The Vogel-Fulcher-Tammann equation is introduced to correlate the viscosity. The experimental results are used to quantitatively analyze the effects on the viscosity of ionic liquid. Toward further understanding the interactions between the ionic liquid and the organic solvents, the viscosity deviation is calculated, and the ideal Grunberg-Nissan equation is performed to check divergence from the ideal. Moreover, Fourier transform infrared spectroscopy (FTIR) is used to study the structure of the sample.