Males of the calopterygid damselfly Neurobasis chinensis (Odonata) have asymmetric genitalia in which the left lateral-process is protruded more than the right process. The function of the lateral-processes are removal of rival sperm from the female spermathecal ducts which is Y-shaped and placed at the deep part of the female sperm storage organ. However, if the spermathecal tube is elongated, it is usually difficult for males to remove sperm stored in this Y-shaped spermatheca, because the male can not use the left and right long lateral-processes at the same time. In this case, males are predicted to use one side of lateral process to insert it into the spermathecal duct. Therefore, the left lateral-process of N. chinensis may be function in removing spermathecal sperm, but the right one is not effective to sperm removal. Such an asymmetric tendency in male genitalia depended on female sperm storage tactics among the calopterygid species: the degree of genital asymmetry is greater in the species with elongated spermathecal ducts in the female than in those with small spermathecal ducts. On the molecular phylogeny of Calopterygidae based on 16S rRNA, asymmetric male genitalia appeared once (all left-handed) and after that the degree of asymmetry changed depending on the female sperm storage tactics.