We employ dewetting behavior of thin polystyrene films to investigate the stability and slippage on the silanized Si wafers with different grafting density of silane brush. PS films are stable on our nonsilanized substrate, but an increase in the grafting density increases the contact angle from zero, causing the appearance and growth of holes. A clear signature of slippage (hole radius similar to times(2/3)) is obtained at low grafting density and low contact angles, which changes to no-slip behavior (hole radius similar to time) on a uniform high density brush. It is also found that the molecular weight of PS does not influence the slippage implying that the slippage observed is caused neither by the entanglement of polymer chains, nor by high non-wet wettability of the substrate, but by the substrate heterogeneity or physicochemical roughness imparted by the incomplete, low density brush with intervening adsorbed PS chains.