This　in　situ　TEM　study　investigated　the　effect　of　sample　thickness　on　the　martensitic　transformation　of　NiTi.　The　transformation　temperature　gradually　decreased　with　decreasing　sample　thickness.　No　transformation　was　observed　when　the　thickness　was　below　a　critical　value.　The　critical　thickness　values　for　the　R　phase　and　martensitic　transformations　were　determined　to　be　similar　to　22　nm　and　similar　to　50　nm,　respectively,　for　the　Ti-50.8　at.%Ni　alloy　studied.　This　apparent　＂size　effect＂　was　attributed　to　the　depletion　of　Ti　in　the　matrix　caused　by　surface　oxidation,　as　confirmed　by　oxygen　content　measurements.　These　findings　demonstrate　that　natural　oxidation　and　its　influence　on　the　transformation　temperature　must　be　taken　into　consideration　in　the　fabrication　of　micro-electro-mechanical　devices　using　NiTi.　(C)　2013　Elsevier　B.　V.　All　rights　reserved.