In polyol processes, it was widely accepted that Ag nanowires (NWs) were formed via uniaxial growth of multiple twinned decahedral particles (MTPs) along the {111} facets. Herein, we show that the above MTP uniaxial growth mechanism for growth of nanorods (NRs) and short nanowires (NWs) is different from that for the growth of long Ag NWs. We provide experimental evidence to show that polycrystalline long Ag NWs (up to similar to 100 mu m) could be formed in high yield (similar to 90%) by a completely different growth mechanism via self-assembly of Ag NPs/NRs. Transmission electron microscope (TEM) measurements show that long Ag NWs are composed of crystalline Ag NPs and NRs with multiple crystal orientations, and many NRs have crystalline structures with pentagonal cross section. Solution phase in situ X-ray diffraction (XRD) measurements show that a strained face-centered tetragonal (fct) phase was gradually formed during the formation and growth of long Ag NWs, in addition to the normal face-centered cubic (fcc) phase. The strained fct phase disappears after partial etching by HAuCl4 and Fe(NO3)(3). The working conditions for the MTP uniaxial growth mechanism and the current nitrate-promoted self-assembly growth mechanism will be compared and discussed.