Pure and Sn or Pt doped $alpha-Fe_2O_3$ thin films were prepared on $Al_2O_3$ substrates by RF-magnetron sputtering method and the sensitivities were compared. It was found that pure $alpha-Fe_2O_3$ thin films did not exhibit much selectivity in CO and $i-C_4H_{10}$ gases while it showed the high sensitivity in proportion to the gas concentration of $C_2H_{5}OH$ gas. Pt-doped $alpha-Fe_2O_3$ showed to be alike sensing properties as pure $alpha-Fe_2O_3$ thin film in $C_2H_{5}OH$ gas. However, Sn-doped $alpha-Fe_2O_3$ thin films exhibited the excellent sensitivity and selectivity in Hz gas. After microstructure modification by plasma etching on pure $alpha-Fe_2O_3$ thin films, the gas sensing characteristics were dramatically changed.