Microscopic structures and mechanical properties of amorphous TiO2 ultra-fine particles have been studied by positron annihilation spectroscopy. A positron lifetime of about 1.5 ns was found as the longest lifetime component tau(3) which is attributed to the ortho-positronium annihilation in open space. The lifetime t3 and the corresponding intensity l(3) was independent of atmosphere: vacuum and various gasses. Accordingly, a micro-void is contained inside the particle as the open space. An intermediate lifetime tau(2) was approximately 0.4 ns. The intensity l(2) was strongly affected by the atmosphere. No effect was found for the particles, the surface of which was coated by silicon dioxide film. This component is arisen from the positron annihilation on the particle surface and the particles contain the surface cracks. When the volume of particles packed in an ampoule was decreased by 1/4 times by compression, the longest component as well as the intermediate one disappeared and only the shortest-lifetime component tau(1) of about 0.24 ns remained. To conclude, the particles are so soft that the micro-voids and the surface cracks are collapsed by compression.