Abstract: The Co doped ATO (Antimony (Sb) doped tin oxide (SnO₂)  composite films were successfully synthesized by using sol-gel combined with dip-coating methods and originating from the primary materials of SnCl₂·2H₂O，SbCl₃ and Co(NO₃)₂· 9H₂O. The structures and properties of Co/ATO composite films were characterized by X-ray diffraction (XRD)，scanning electron microscope (SEM)，four-probe method，ultraviolet visible near-infrared spectrophotometer and vibrating sample magnetometer (VSM). The results indicate that a tetragonal rutile structure is retained by a little Co doped ATO，and the nano-particles are well distributed in the thin films and their size can be controlled in the range of 4～7 nm. With increasing the Co content，the resistivity of the composite films decreases and then increases，and it presents a very low value at 0.5 mol% Co. In the range of 400～700 nm visible region，the transmit tance values are up to 90 %，also the transmit tance in the visible region and the optical band gap trends to decrease with increasing the Co content. The room temperature ferromagnetism of the composite film has a coercive force of about 100 Oe when the Co-doped is 3.0 mol%，and it is mainly originated from the interaction of magnetic atoms and the body carrying ions.