The molecular configuration of isotactic PMMA Langmuir-Blodgett (LB) films was investigated with scanning tunnelling microscopy (STM). STM images of the isotactic PMMA LB films, transferred at the surface pressure of 15 mN m-1, show a closely packed well-ordered arrangement in which two phases with helical and linear chain configurations are mixed. The appearance of the STM images and the width and pitch of the helix indicate that the transition at the surface pressure of 8 mN m-1 corresponds to formation of the right-handed 5, single-strand helix. The transformation process can be explained by a one-dimensional growth.