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 ∼22 nm and ∼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. © 2013 Elsevier B.V. All rights reserved.