The Co doped ATO (Antimony (Sb) doped tin oxide (SnO
2) composite films were successfully synthesized by using sol-gel combined with dip-coating methods and originating from the primary materials of SnCl
2·2H
2O,SbCl
3 and Co(NO
3)
2· 9H
2O. 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.