ATO(Sb doped $SnO_2$) thin films whose thicknesses were 600, 1100 and $2100AA$ were prepared by DC magnetron sputtering method. They showed the lowest resistivity at DC sputtering power 0.24kW and had lower resistivity with increasing thickness. The power dependence of resistivity among ATO thin films was also different with thickness. The increase of carrier concentration in ATO thin films was responsible for the decrease of resistivity with thickness increase. ATO thin films which were prepared at 30sccm oxygen flow rate showed a great change of sheet resistance under 1M HCl solution. The investigation of SAM(Scanning Auger Microprobe) revealed that oxygen atomic percentage on the surface of ATO thin films was changed. The decrease of sheet resistance also occurred when ATO thin films, prepared at 30sccm oxygen flow rate, were exposed to air for a long period of time. For this reason, it was considered that the desorption of oxygen on ATO surface was accelerated by HCl.