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The magneto-transport properties of thin single crystal Bi films epitaxial grown on Si (111)-7 × 7 surfaces are investigated systematically as functions of film thickness (5-55 nm) and temperature.Under a perpendicular magnetic field,the positive magnetoresistance (PMR) effect is normally found,and its curve shapes are evolved systematically with film thickness.In contrast,under parallel magnetic fields the PMR effect observed for thinner Bi films develops into the negative magnetoresistance effect with the increasing magnetic field for the thicker Bi film.Our analysis indicates that there exists strong competition between the weak anti-localization effect in the surface states and the weak-localization effect in the bulk states of the Bi film,which induces the anomaous changes in the parallel magneto-resistance curves.The temperature-dependent experiments further demonstrate that the surface state plays an important role in the magneto-transport process of Bi films.