The optimum film thickness l* and optimum grain size r0* for sensitivity in nano metal oxide gas sensing thin film are existent, and the value is dependent on the material and technics. The existence of optimum grain size is a new kind of mesoscopic effect, which is from the disability of Fermi statistics for negative oxide ions when the grain size reduced to a certain extent, according to the modification of Kubo theory for the grains keeping neuter to fermi statistics when the grain size reduced to character length rm , we get that r0* =2rm, and the values of rm and optimum grain size r0* are estimated with the word function in Kubo theory.