The highest etch rate of Pt film was obtained at 10% $Cl_2$/90% Ar gas mixing ratio in our previous investigation. However, the problems such as the etch residues(fence) remained on the pattern sidewall, low selectivity to oxide as mask and low etch slope were presented. In this paper, the etching by additive $O_2$ gas to 10% $Cl_2$/90% Ar gas base was examined. As a result, the fence-free pattern and higher etch slope as about 60$^{circ}$was observed and the selectivity to oxide increased to 2.4 without decreasing of the etch rate $1500{AA}$/min. XPS surface analysis proved that a only little $O_2$ gas removes the Pt-CI compounds as residues on the etched surface.