We are performing systematic observation studies on the Galactic interstellar isotopic ratios, including 18O/17O, 12C/13C, 14N/15N and 32S/34S. Our strategy focuses on combination of multi-transition observation data toward large samples with different Galactocentric distances. Our preliminary results show positive Galactic radial gradients of 18O/17O and 12C/13C. In both cases, the ratio increases with the Galactocentric distance, which agrees with the inside-out scenario of our Galaxy. Observations of other isotopes such as 14N/15N and 32S/34S are on-going.
