Highly ordered porous anodic aluminum oxide (PAA) membranes are prepared by electrochemical method. By alternating current electrodepositing, Co nanowire arrays are fabricated in the porous anodic alumina (PAA) in an electrolyte CoSO4. The morphology and microstructures of Co/PAA are characterized by SEM and X-ray diffraction. The results show that Co nanowires are indeed synthesized in porous alumina, and the nanowires have a preferred crystal face (100) during growing. The optical system is established in which the changes of polarization state for sample can be measured. In this optical system, the depolarization effect in Co/PAA which often be used in optical isolator is measured in large variation range of adjustable magnetic field. The experiment result shows that Co/PAA has depolarization effect in adjustable magnetic field, and the polarization performance is best in Co/PAA when it reaches saturated magnetization. The depolarization effect is occured when the intensity of magnetic field increases over saturation intensity. The principle for magnetic depolarization effect is also given according to the experimental result.