A common size effect mechanism exists in fracture of vastly different materials with huge differences in size. The common mechanism is governed by the interaction of the critical crack, surface condition and material microstructures. The distance of a crack tip to the surface thus becomes an important measurement, which can either be the crack size itself or uncracked ligament depending on whether a shallow or deep crack is considered. The surface roughness and surface flaws form an effective boundary layer in micro-specimens/structures such as thin films and MEMS structures regardless whether they are amorphous, or have extremely fine micro-structures. The boundary layer in macro-specimens such as polycrystalline ceramics and concrete is determined by the material micro-structures and fracture process zone, which is also determined by the material micro-structure. The common size effect mechanism is then linked to the thickness or depth of the boundary layer and its ratio to the crack length or uncracked ligament.