A seat slide mechanism for vehicles includes a screw member extending longitudinally within an interior portion of the upper rail and a nut member engaging with the screw member. The nut member extends downwardly between the opposed walls of the upper rail to the base of the lower rail to form separated spaces at the both sides of the nut member and between laterally elongated slide members of the upper rail and the base portion of the lower rail. These separated spaces are helpful in arranging the rollers without any interference with the nut member or the screw member.