Monodispersed polystyrene with different molecular weights, M-w, from below the entanglement molecular weight, M-e (similar to 20K Da), to well above was studied for effects of chain ends and chain entanglement on the glass transition temperature, T-g, of polymer thin films. The relative importance between these two effects is expected to reverse as M-w crosses over M-e. We found that the T-g generally decreases with decreasing film thickness, t, consistent with previous findings. The difference between results from the M-w = 13.7K and the 550K samples was negligible, suggesting that entanglement has little significance on the thickness dependence of the T-g of polymer under confinement in thin films. On the other hand, the Mw dependence of T-g was markedly weakened compared to that of the bulk when t was reduced to 15 nm. According to Fox and Flory's theory, our result reveals that some of the chain ends in the film have been segregated to the surface, but they have little effect on the T-g of the film. Our findings undermine a surface mobile layer where the surface chain ends localize being the cause for the reduction of T-g observed.