A water pump of an engine cooling system includes a thermal valve that divides an outlet space of a pump housing of an outlet port, through which the engine coolant pumped into the internal space of a pump housing is discharged, into a front end discharge region and a rear end discharge region. The thermal valve also forms a dual flow path through which the engine coolant is discharged to the front end discharge region and the rear end discharge region according to the increase of the coolant temperature. The cooling of a cylinder block and a cylinder head of an engine are thus separately adjusted.