An ink composition useful in an oil-based ballpoint pen capable of drawing a satisfactory line on the surfaces of various materials to be written which are not only papers but also non-permeable materials such as glasses and plastics as well as fibrous materials such as fabrics is provided. Such a ink composition comprises a colorant, an organic solvent having a vapor pressure of 5.0 to 50 mmHg (20.degree. C.) as a main solvent, such as n-propyl alcohol, isopropyl alcohol and propylene glycol monomethyl ether and a shear-viscosity reducing agent.