The effect of film thickness on the organization kinetics of ultrathin (50-1000 Angstrom thick) spin-cast polymer films was studied using poly(di-n-hexylsilane) and UV absorption spectroscopy. We found an extensive reduction in the crystallinity as well as a reduction in the rate of crystallization for film thicknesses below 500 Angstrom resulting from the constrained geometry. Modeling using polymer crystallization theories elucidated the surface-induced phenomena. We found that the dimensionality of the growth depended both on the film thickness and on the crystallization temperature. At low crystallization temperatures (below 0 degrees C) and for films thicker than 220 Angstrom, the nucleation is bulk and However, at higher crystallization temperatures (above 3 degrees C) and for low film thickness (below 150 Angstrom), the growth is one-dimensional and heterogeneous nucleation becomes important. For 500 Angstrom thick films, the transition between the two nucleation regimes occurs abruptly around 3 degrees C.