星际介质中同位素丰度比１８Ｏ／１７Ｏ可以很好地示踪星际介质中元素增丰来自大质量恒星与中小质量恒星的相对抛射量．为了确定银河系不同区域星际介质中的该同位素丰度比，笔者利用ＪＣＭＴ-１５ｍ望远镜，对１７个不同银心距（１～１６ｋｐｃ）恒星形成区的Ｃ１８Ｏ和Ｃ１７Ｏ分子的Ｊ＝２－１跃迁谱线进行了观测．在其中的１１个源，同时探测到了Ｃ１８Ｏ和Ｃ１７Ｏ分子谱线．分析得到了该样本的同位素丰度比１８Ｏ／１７Ｏ，发现该丰度比存在一个明显的径向梯度，即该丰度比随着银心距增加而增加，这和之前Ｃ１８Ｏ和Ｃ１７ＯＪ＝１－０谱线的观测结果是一致的．但是，由Ｊ＝２－１谱线得到的同位素比值普遍低于Ｊ＝１－０的比值．这应该是由于Ｃ１８ＯＪ＝２－１谱线比其Ｊ＝１－０谱线的光厚效应更严重导致的．这也意味着在确定样本的丰度比时，光深效应是不可忽视的．后续对大样本的多条跃迁谱线观测，对准确确定丰度比及其梯度分布非常重要．由观测得到的丰度比梯度越准确，就越能在理论模型研究中对ＣＮＯ同位素的合成进行更好的限定．

The interstellar isotope ratio of 180/"70 can reflct the relative amount of the secular enrichment by ejecta from high-mass versus intermediate-mass stars. We present observations of Cl*O and C"OJ=2-1 lines toward 17 star forming regions with different galactocentric distances through James Clerk Maxwell ( JCMT-15m) telescope. Our sources cover the galactocentric distance range of1 - 16 kpe. Among them, 11 targets were de- tected with both Cl8 0 and c'"O lines. Our analysis results show an obvious radial gradient of 180/"70 along the galactic disk, which was proposed from J = 1 - 0 spectral line observations. However, the isotope ratios ol 80/"0 derived from J =2 -1 lines are smaller than the J=1 - 0 ratio in general. This may be caused by more optically thick effect of Cl8O J=2-1, with respect toitsJ=1 -0 line. It reflects further that the optical depth effct toward our molecular clouds couldn't be ignored, to determine accurate abundance ratio. Further obser- vations for multi-transition lines of C8O and c'O toward a large sample are important to determine accurately this radial gradient. The more precise the 180/'0 gradient determination, the tighter the constraints we can im- pose on the synthesis of CNO isotopes.