The fabrication and structural characteristics of ultrathin nanocrystallization silicon films is reported. Amorphous Si layers, with nominal thicknesses in the range of 3.5-11 nm, were deposited by low-pressure chemical vapor deposition onto amorphous SiO2 and then crystallized using rapid thermal annealing at temperatures from 650 to 750 degrees C. High-resolution electron microscopy revealed that the resulting films were comprised almost entirely of Si nanocrystallites with a small fraction of remaining amorphous material. The grain size in the vertical growth direction was controlled by the thickness of the as-deposited amorphous Si film, whereas the lateral grain size was determined by appropriate choice of the annealing conditions.