The capacity of organisms to change their sex has evolved independently in several species. Sex change has been widely studied, but research approaches have differed and conclusions have often been specific. Although sex allocation theory provides a unifying framework for the study of sex change, this unity has not always been appreciated, Environmental sex reversal (ESR), which results in a mismatch between genotypic and phenotypic sex, is well documented in numerous species and may be induced by chemical exposure. In the present study, an attempt was made to hypothesize the phenomenon of sex reversal in both Asian species (Tachypleus gigas and Carcinoscorpius rotundicauda) of the horseshoe crab in accordance with the size of the animals. Our findings suggest that both Asian species of horseshoe crabs ultimately show a positive sign of sex reversal phenomenon, however, this hypothesis needs further confirmation by studying the hormonal changes at all moulting stages of the horseshoe crab