Interface-enhanced high-temperature superconductivity in one-unit-cell(UC)FeSe film on SrTiO3(001)(STO)substrate has recently attracted much attention in condensed matter physics and material science.Here,using ex situ transport measurements,we report on the superconductivity and linear magnetoresistance in FeSe ultrathin films with different thicknesses on STO substrate.We find that the onset superconducting transition temperature(Tc)decreases with increasing film thickness of FeSe,which is opposite to the behavior usually observed in traditional superconductor films.By systematic post annealing of 5 UC FeSe films,we observe an insulator–superconductor transition,which is accompanied by a sign change of the dominant charge carriers from holes to electrons at low temperatures according to the corresponding Hall measurement.We also observe a linear magnetoresistance which occurs at the superconducting region regardless of varied film thickness and protection layers.The linear magnetoresistance exhibits a significant anisotropy with the direction of the applied magnetic field.