Decay kinetics of two-dimensional islands and holes with diameters of a few micrometers to ∼100 nanometers on Si(111) was studied by low-energy electron microscopy close at the “1×1”-to-7×7 phase transition temperature. It is shown that the decay is governed by diffusion rather than attachment/detachment of the atoms at steps. Comparison of the decay rates of island and hole indicates that there is an upper limit for the Ehrlich-Schwoebel barrier. We also show that the surface mass diffusion constant on 7×7 is smaller than that on “1×1”. Slower surface mass diffusion on 7×7 than on “1×1”, coupled with preferential formation of 7×7 on the upper terrace of a step, produces a diffusion barrier during the phase transition, resulting in an asymmetry in the thermal decay rate between island and vacancy island. The diffusion barrier also causes the mass flow induced by the atom density difference between “1×1” and 7×7 asymmetric in the step-up and step-down directions.