A side sill assembly of the vehicle includes a side sill inner, a side sill outer, and a reinforcement unit provided between the side sill inner and the side sill outer. The reinforcement unit includes an upper member provided in a longitudinal direction of the vehicle; a lower member provided in the longitudinal direction of the vehicle, and located to be spaced from the upper member under the upper member; and an intermediate member having an upper surface bonded to a bottom portion of the upper member and a lower surface bonded to an upper surface of the lower member alternately formed thereon.