A cooling system of a cylinder liner of an internal combustion engine includes a plurality of first passages formed on an outer surface of the cylinder liner of the internal combustion engine and spaced apart from each other. A second passage is formed on the outer surface of the cylinder liner and runs in an axial direction of the cylinder liner, coupling the first passages to each other. The second passage receives, via an inflow part thereof, a coolant supplied in a traverse direction substantially perpendicular to the axial direction. A third passage is formed on the outer surface of the cylinder liner, and runs in the axial direction of the cylinder liner, coupling the first passages to each other. The third passage outputs the coolant received from the first passages to an outflow part thereof. The second passage has a traverse-sectional area which decreases as a distance of the traverse-sectional area from the inflow part increases. The third passage has a traverse-sectional area which decreases as a distance of the traverse-sectional area of the third passage from the outflow part increases.