Cobalt nanowires were fabricated by alternating current electrodeposition using anodic porous alumina template. Diameter of the pores was controlled by the pore widening time. Effect of pore widening time on the magnetic properties and microstructure of the nanowires was investigated. High-resolution scanning transmission electron microscopy was used to study the morphology of the pores as well as nanowires. Magnetic properties were measured by vibrating sample magnetometer at room temperature, and were found to be size-dependent. When the field was applied parallel to the nanowires, coercivity of the nanowires reaches to the maximum value ∼ 2200 Oe for 10 min pore widening time. Coercivity decreases linearly with further increase in the pore widening time. In the case, when the field was applied perpendicular to the nanowires, coercivity was found to decrease monotonically when the pore widening time was increased from 0 to 10 min. © 2009 IEEE.