Thermal relaxation of isolated single silicon two dimensional (2D) islands, 2D vacancy islands and pyramid type islands, produced on the Si(111)(7×7) surface by using a tip of a scanning tunneling microscope are observed by temperature variable scanning tunneling microscopy. Thermal decay rates of silicon islands are measured at various substrate temperatures of 650 K-850 K. Effects of the probe tip on the rates are measured and reduced rates are determined without the tip effects. The sizes of islands depend on time with a function form of (t₀-t)^α. For single bilayer islands, α was 1 without the tip effects and 2/3 with the tip effects. From these results, we conclude that the intrinsic decay process of islands is attachment-detachment limited but is diffusion limited with the tip effect. Activation energies for the intrinsic decay of single 2D islands and 2 D vacancy islands are 1.5 eV and 1.3 eV, respectively. Characteristic (5×5) islands with long life times are found during decay. The (5×5) structure is also observed on pyramid type island surfaces. We discuss the results in terms of two dimensional vapor phase processes.