Fish olfactory system is composed of olfactory sac, olfactory nerve and olfactory bulb. Olfactory sac, also called olfactory organ, is composed of olfactory epithelium and central core. Chemical signals are first detected by the olfactory receptor neurons that randomly distribute in the entire olfactory epithelium, and then transferred to the olfactory bulb through the olfactory nerve. There exist different functional regions in the olfactory bulb, where the chemical signals are integrated and transferred to the telencephalon to play physiological functions. The sex pheromones play their functions through the olfactory system in fish reproduction, and the sex pheromonal receptors of the olfactory neurons play an important role. Usually, two approaches are used to investigate the sex pheromonal receptors in fishes: the first is based on species-specific electrophysiological or behavioral responses to sex pheromones at very low concentrations, and the second is based on the conservative structures of receptors genes taking reference of the mammalian counterparts.