Co-doped tin oxide thin films were prepared using spin-coating method. Variation of doping content on the magnetic property and optical property were measured. XRD and magnetic measurement shows that Co solubility limit in SnO2 is less than 3%. When the doping content is lower than 3%, the films show good optical transmittance. When the doping content is reached to 10%, the optical transmittance became lower. When the solution is diluted, the optical transmittance drops more slowly.