There is provided a slide door structure of a vehicle which has a slide door to slide in a vehicle longitudinal direction at a side of a vehicle body, comprising a center rail which extends in the vehicle longitudinal direction at the side of the vehicle body so as to guide a roller provided at the slide door, and an expansion preventing bracket which is provided at a front end portion of the center rail so as to connect a pair of guide-face portions which face to each other for preventing an expansion of a center-rail end. Accordingly, the expansion of guide-rail end can be prevented in case the outward load acts on the slide door when the slide door is in the closed state.