A reinforcement structure of a side sill for a vehicle may include a side internal panel disposed within a vehicle body; a side sill reinforcement attached to the external surface of the side internal panel and forming a sill space; and a side sill extension closing the opening portion open to the front of the sill space formed by the side internal panel and the side sill reinforcement and being coupled to the side internal panel, the side sill reinforcement and a side sill reinforcing member, respectively.