We successfully prepared SnO2 nanoparticles of two different sizes by hydrothermal synthesis and calcination at 400 degrees C to evaluate the oxygen adsorption equilibrium constant K(O2-ad) on a particle surface. The K(O2-ad) values were calculated from the relationship between the electric resistance and the oxygen partial pressure (P-O2). The K(O2-ad) values of 7 and 17 nm SnO2 nanoparticles were almost in the same range when the donor densities of these nanoparticles are the same. Additionally, the sensor responses of these nanoparticles to hydrogen were affected by the surface-area-to-volume ratio because the K(O2-ad) values are the same. Thus, we propose that the K(O2-ad) value can be evaluated on the basis of the relationship between the electric resistance and P-O2.