This paper investigates seasonally averaged atmospheric CO2 source or sink signals observed at Waliguan Baseline Observatory (WLG, 36¿17'N, 100¿54'E, 3816 m asl) in western China from 1991 to 2002. Both linear and geometric mean regressions as well as statistical significance test were performed between detrended CO2 and stable isotope monthly data by means of Keeling Model and Miller-Tans Model. The estimated slope Δδ13C/ΔCO2 due to the seasonality in CO2 and δ13C is (-0.050 ¿ 0.004)? ppm-1, the mean source/sink isotopic signatures δs (i.e., δ13Cs and δ18Os) are (-26.159 ¿ 1.924)? and (-7.662 ¿ 2.113)?, respectively, and the mean atmospheric δ13C discrimination δdis (i.e., δs minus δbg) where the δbg is the isotopic value of the background atmosphere was (-18.174 ¿ 1.959)? by the Keeling Model, in agreement with results from other continental background sites in the Northern Hemisphere. We suggest that exchange with terrestrial biosphere dominates the observed CO2, δ13C and δ18O seasonal cycles at WLG. In addition, atmospheric CO2 and δ13C data from 11 selected NH sites in the NOAA ESRL air sampling network from 1998 to 2002 were analyzed and compared to the WLG data for the same period to better address common and specific features observed in this region. The annual cycle amplitude differences, secular and seasonal Δδ13C/ΔCO2 discrepancies among sites will be useful to better understand carbon uptake and release especially on the Eurasian continent. The estimated δs during certain times at each specific site could possibly provide useful information on CO2 fluxes.