According to the pedogenetictheories, soils in similar soil landscapes, like climate, parent material, landforms, plants, animals’ activities,are supposed to have similar characteristics as a result of pedogenesis. However, there are no soil pedonsabsolutely the same, even though they have developed in the same landscape, for their developments could have been drastically affected by various soil forming factors. Specially, in classification of basic soil species in the soil geographico-genetic classification and of basic soil series in the soil taxonomy, a specific or diagnostic soil horizon is cited to differentiate soil structures. Yet, the specific soil horizon diversifies and is still blurred in specification. Diagnostic horizons were created and used for the first time during the 2nd national soil survey. Then the concept and the nomenclature of diagnostic horizons are accepted and used in classification of basic soil series in the soil taxonomy for the purpose of distinguishing structures of soil profiles. As diagnostic horizons play such a crucial role in interpreting structure of a soil series, it is necessary to redefine and collate them systematically. In this paper, soil seriestypicalof Henan were cited as examples, of which diagnostic horizons involved in classification of soil series were collated and summarized. They have been subjected to clayification, calcification, anthrogenetic processes, humic accumulation, etc. during their development. So in terms of the cause of genesis, diagnostic horizons could be sorted into 3 groups, i.e. those formed during the soil genesis under modern soil forming environments, those formed under soil forming environments of geological origin or geologic epoch, and those under human activities. On such concepts of soil series, a classification and retrieval system is brought forth for diagnostic horizons with a view to making the concept of diagnostic horizons, more systematic, more comprehensive, opener and easier to apply.