Abstract: Three hypotheses is put forward, based on microexamination about sample evaporation during determaination of isotope abundance by thermal ionization:1. Only atoms uncovered by outer layer atoms can be vapoured; 2. Atoms covered by outer layer atoms can diffuse but cannot vapoure; 3. The probability of diffusion of isotope atoms is in direct proportion to its probability of evaporation. The differential eguations and boundary conditions are established, based on these hypotheses. The theoretical formulary about sample atom number with different diffusion coefficient are reasoned out; the relationship between the isotope abundance ratio and the sample wastage of melting and solid samples and the samples simultaneously at evaporation and decomposition are discussed in theory. The study indicates that the relationship between the sample isotope abundance ratios and the sample wastage with diffusion coefficient >10 -15 m 2/s is accordant to the discuss of H.Kanno; the sample isotope abundance ratio with diffusion coefficient <5×10 -21 m 2/s is independent of sample wastage and is equal with real value; the isotope abundance ratio of the samples simultaneously at evaporation and decomposition is related to the ratio of dissolution and evaporation.