We present experimental results for the glass transition behavior of polystyrene (PS) films on grafted PS layers of the same chemical identity as a function of film thickness. Our results suggest that the T-g of PS films on brush substrates decreases with decreasing film thickness. The thickness dependence of T-g was observed to be more pronounced for the films on the shorter brushes with the high grafting density. We propose a qualitative rationalization of the observations by invoking both interfacial energy considerations as well as by adapting the percolation model for the glass transition of polymer films.