PtSi/Si contacts were studied as a function of thickness. A change of Schottky barrier height up to 100 mV from its bulk value was detected when PtSi thickness decreased to 6 nm. One explanation is the quantum size effect, generally confirming theoretical predictions. This effect also results in an increase of contact resistance in Ohmic PtSi/p-Si contacts. Furthermore, the nanoscale PtSi thin layer presented a poor electron screen from the outside contact layer of Al. It was found that for PtSi > 50 nm, the contact properties of Al/PtSi/Si is unchanged by the presence of the Al layer.