We have investigated the optimum condition of the two-dimensional epitaxial recrystallization of an amorphous Si layer grown on clean Si (100) substrates aiming at the formation of the patterned silicon-on-insulator structure. The amorphous Si layers were firstly deposited on the clean flat crystalline Si substrates with ~15 nm thick using an electron bombardment evaporator in the ultrahigh vacuum environment at room temperature (RT) to 550C. The deposited Si layer was fully crystallized during deposition at substrate temperature of 415C. We then attempted the recrystallization of the RT-deposited samples by ion beam irradiation at substrate temperatures of 300 to 700C. The RT-deposited Si layer was recrystallized with about 12 nm by ion beam irradiation at 300 to 700C. It is supposed that the crystalline grains of (100) orientation were preferentially formed in the amorphous layer by ion beam irradiation.