We have investigated the notch width dependence of the domain wall structure by measuring the wall resistances in Co/Ni nanowires with perpendicular magnetization. Wall resistance was abruptly increased below a notch width of 43 nm. By comparing the experimental results with theoretical calculations of wall resistance and the micromagnetic simulation of wall energy, this increase in wall resistance was found to be caused by the change in the wall structure from the Bloch to Neel wall caused by the decreased notch width. (C) 2011 The Japan Society of Applied Physics