A systematic study is reported on the thickness dependence of the electrical resistivity in thin films of the giant magnetoresistance manganite Nd2/3Sr1/3MnO3. We observed a first-order phase transition versus thickness in these films, which is seen as a jump of about 30 K in the metal-to-insulator transition temperature (Tp) at film thickness of 50–60 nm. This phenomenon is attributed to a sudden release of strain in the film as its thickness increases. We also observed at low temperatures, 5–30 K, another transition from localized-to-metallic behavior versus film thickness, which is also related to the strain relief in the films. © 2000 American Institute of Physics.