Despite the several decades study of the confinement effect of the polymeric nanomaterials, how the confinement influences 1D polymeric fiber nanomaterials is little understood. Here, we report that confinement can render ultrathin polymeric fibers rigid. By observing the changes in the crystalline and amorphous morphologies of electrospun nylon-6 nanofibers with variations in diameter and shape, we reveal that their crystalline phase changes into highly packed, stable alpha phase when the diameter is smaller than 120 nm. In addition, the molecular motion of the amorphous chains is severely suppressed with decrease in nanofiber diameter, indicating that the amorphous chains are also closely packed, forming a rigid structure. Indeed, the change in chain topology by confinement suppressed the release of rhodamine B from the ultrathin nanofibers. These findings allow us new insights for the design and development of advanced 1D polymer nanomaterials.