Female pleopods in Procambarus clarkii have a function of holding eggs, and their length shows positive allometry. It is likely that the positive allometry has resulted from directional selection in terms of egg holding ability. Firstly, we examined the allometry of reproductive potential (ovarian egg number). The allometric value of reproductive potential was lower than that of pleopod length, which indicates that the positive allometry of pleopod length is not merely the reflection of the allometry of reproductive potential. Secondly, we examined the relationship between the relative length of pleopod and egg holding ability to test the presence of directional selection on the pleopod length, and then compared the strength of the directional selection between the individuals differing in their body sizes to infer the mechanisms causing the positive allometry for female pleopods. There was a positive relationship between relative pleopod length and relative egg mass remaining to the pleopod, and this relationship was stronger in small individuals. These relationships were not merely the reflection of relationship between pleopod length and reproductive potential. These results indicate that directional selection was acting on the length of female pleopods, and the strength of the directional selection was stronger in smaller individuals. Therefore, it appears that the positive allometry of female pleopods in P. clarkii has been caused by the constraint for possessing longer pleopods in small individuals, although the mechanisms of the constraint remain unknown.