A gas distribution assembly provided in an apparatus for treating a substrate with plasma to distribute gas includes a gas distribution plate formed with a plurality of gas introduction holes for diffusing gas supplied from the gas supply unit; a shower head plate disposed at the upper portion or lower portion of the gas distribution plate to be in contact with the gas distribution plate and having a plurality of gas supply holes formed at positions communicating with the gas introduction holes to penetrate through the upper surface and the lower surface; and a fastening member provided at a side surface in contact with the gas distribution plate and the shower head plate and including a first coupling portion coupled to the gas distribution plate and a second coupling portion supporting the lower surface of the shower head plate to contact the gas distribution plate and the shower head plate.