Abstract: A total of 129 mature individuals of the giant freshwater prawn Macrobrachium rosenbergii (65 females and 64 males) were collected.Morphometric parameters of rostrum length,carapace length,carapace width,carapace height,abdomen length,body length,second cheliped length,total length and body weight between male and female prawns were measured to study sexual dimorphism of Macrobrachium rosenbergii.Body length and body weight of mature male prawn were significantly larger than female prawn through independent sample t-test(P<0.001).Sexual dimorphism index of carapace length,carapace width,carapace height,abdomen length,second cheliped length and body length between male and female prawns was above 1 indicating that male prawn was larger than female prawn.The differences were not statistically significant of rostrum length between sexes by One-way ANCOVA(F=1.279,P=0.260), there are significant differences of carapace length,carapace width,carapace height,abdomen length and second cheliped length between male and female prawns by using One-way ANCOVA(P<0.01). Analysis of the carapace length-body length,carapace width-body length,carapace height-body length and second cheliped length-body length relationship of male and female Macrobrachium rosenbergii revealed that males grew faster than females,but abdomen length-body length relationship of males and females indicated that female's abdomen grew faster than males.Sexual selection allowed males to have a larger body size and second cheliped to ensure that they could effectively defeat competitors in reproductive competition and enhance mating success.