A side sill rigidity reinforcement structure of a vehicle may include an external side sill external member which includes an upper support portion and a lower support portion each having a section convexly formed outwardly from a vehicle and mounted vertically, is formed to connect the upper support portion and the lower support portion; an internal side sill external member which is mounted inside any one of the upper support portion and the lower support portion; a side sill external panel which is bonded outside the external side sill external member; and a side sill internal panel which has an upper portion and a lower portion bonded to the side sill external panel.