Cluster ion implantation has a great potential for semiconductor manufacturing, such as ultra shallow junction formation and strain engineering of channel. Stress in Si implanted with carbon cluster ions (C7H 7) was much higher than that in Si implanted with carbon monomer ions. Multiple collisions of atoms occurred during cluster ion implantation, and a fully amorphized Si layer can be formed. The stress in the implanted layer strongly depends on the annealing condition. There are two different directions for the stress. One is parallel to the surface, which is usually measured with conventional X-ray diffraction. The other is perpendicular to the surface, which can be measured with in-plane X-ray diffraction technique. The stress perpendicular to the surface was about 50% of the stress parallel to the surface even after flash lamp annealing.