The range of viscosity ratios suitable for the fabrication of core-shell fibers was studied using a mixture of poly(methyl methacrylate) and polyacrylonitrile as a test system. The effect of the viscosity ratio of the outer/inner solutions on the morphology of the obtained core-shell fibers was studied by altering the concentrations of the two polymer solutions. The different viscosity ratios of the outer/inner solutions affected the uniform core-shell structure formation, overall diameter and wall thickness of the obtained fibers, which was confirmed by scanning electron microscopic and transmission electron microscopic observations. It was found that a viscosity ratio of the outer/inner solutions ranging from 1.22 to 2.82 resulted in uniform core-shell morphologies of the resultant electrospun fibers.