A side sill assembly for an eco-friendly vehicle protects a high voltage battery mounted therein from shock upon side collision. The side sill assembly includes: a center member 11 fastened to a side surface of a center floor panel 23 for a vehicle, a front outer member 12 fastened to a front end of the center member 11, and a rear outer member 14 fastened to the rear end of the center member 11, in which each of the center member 11, the front outer member 12, and the rear outer member 14 has a cylindrical cross section and at least one partition wall for partitioning the inside thereof horizontally and vertically to absorb collision energy upon side collision.