A bottom weight device of a roller blind is primarily composed of two rails and two side covers, wherein the two rails are roughly in a U-shape and are provided with longitudinal boreholes, and inner rims of the two side covers are provided with lock lumps corresponding to the boreholes of rails, with the two lock lumps being separated by a proper distance, thereby constituting a bottom weight device after inserting and fixing the lock lumps of two side covers into the boreholes of rails. Accordingly, a bottom end of the roller blind can reeve around the gap between two rails, and the roller blind can move up and down more smoothly and steadily, and can be prevented from being lifted by a wind blow.