  The isotopic ratios are good tools for probing the stellar nucleosynthesis
and chemical evolution. We performed high-sensitivity mapping observations of
the J=7-6 rotational transitions of OCS, OC34S, O13CS, and OC33S toward the
Galactic Center giant molecular cloud, Sagittarius B2 (Sgr B2) with IRAM 30m
telescope. Positions with optically thin and uncontaminated lines are chosen to
determine the sulfur isotope ratios. A 32S/34S ratio of 17.1\pm0.9 was derived
with OCS and OC34S lines, while 34S/33S ratio of 6.8\pm1.9 was derived directly
from integrated intensity ratio of OC34S and OC33S. With independent and
accurate measurements of 32S/34S ratio, our results confirm the termination of
the decreasing trend of 32S/34S ratios toward the Galactic Center, suggesting a
drop in the production of massive stars at the Galactic centre.
