Nadezhdiella cantori (Hope) is an important longhorn pest of citrus trees in Asia. The sex ratio was male-biased (≈1 female:1.5 males) on the mating and oviposition site. Males were significantly smaller than females but had significantly longer antennae and greater slope for the linear antennal-body relationship than females, suggesting that males are subject to higher sexual selection pressures than females, and their antennal length is the highly selected phenotypic character. Males aggressively competed for mates and guarded females through prolonged pair-bonding. During pair-bonding, males performed a full-mounting when mating and a half-mounting when resting or during female oviposition. Larger males with longer antennae had an advantage over smaller ones with shorter antennae in both fighting and mating success, but the slopes for linear antennal-body relationships remained similar for all males. It is suggested that the male antennal and body lengths may have evolved symmetrically for both mating and fighting advantages. Larger females did not have greater probability of being mated, but females with greater slope for the antennal-body length relationship were more likely to be chosen by males for mating. The allometry in the female antennal length may be a phenotypic indicator of female reproductive fitness, with greater slope for antennal-body length relationship representing higher quality. After the initial fight, the male loser seemed to be able to remember the winner and retreated immediately without the occurrence of another fight after encountering (contacting) the previous victor. Such memory lasted for at least one night.