The effect of the film thickness of C49 TiSi2 on the nucleation of C54 TiSi2 was investigated using X-ray diffraction and transmission electron microscopy. The C49 films of different thicknesses were prepared by isothermally annealing Ti/polycrystalline Si bilayers of varying Ti thicknesses (250, 550 and 1000 Angstrom) in a high vacuum furnace. Nucleation studies were carried out in the temperature range from 640 to 740 degrees C. It is found that the transformation temperature increases with decreasing initial C49 film thickness. The microstructure and morphology of the C49 phase depend upon the Ti film thickness and significantly influence the nucleation behavior of the C54 phase. An anomalous nucleation mode transition is observed for the C54 TiSi2; for thicker films (starting with 1000 Angstrom Ti), the C54 TiSi2 nuclei predominantly form at normal grain boundaries (two-grain junctions) of the polycrystalline C49 phase, while the C54 phase nucleates at grain edges (three-grain junctions) of the C49 disilicides for samples starting with 250 and 550 Angstrom Ti. The physical significance of these observations associated with the thickness scaling is discussed in terms of energy consideration of nucleation at different geometrical sites, nucleation site density and largely increased surface-to-volume ratio.