According to method for estimatingspecies richness introduced from modern biology, this paper took four localities from the Cathaysiorthis brachiopod fauna as an example. Their underlying diversity was estimated by the software Estimates. By means of curve fitting, the calculation of sample size corresponding to a given diversity was made, which can provide important information for future sampling. Such diversity estimation is a new field in palaeontology and is promising in comparison and estimation of diversity in specific fossil assemblages. The paper suggests that rarefaction should be accompanied by the estimation method to obtain the most comprehensive extent of diversity information.