1-Butyl-3-methylimidazolium chloride ([BMIM]Cl) was applied as solvent to dissolve the cellulose diacetate (CDA) directly. The solution rheology of CDA in [BMIM]Cl was investigated by steady shear and oscillatory shear measurements. In the range of the concentration and temperature studied, CDA/[BMIM]Cl solutions showed the typical shear-shinning behavior and the viscosity decreases with increasing temperature. Compared with the cellulose solutions in [BMIM]Cl, the smaller apparent viscosity of CDA/[BMIM]Cl was shown, which mainly resulted from the weakened interaction between substituted cellulose chains and ionic liquid. The deviation of Cox–Merz rule with steady shear viscosity smaller than complex viscosity in the whole deformation rate range we studied indicates that entangled CDA/[BMIM]Cl solutions are not homogeneous solution systems.