To explore the potential role of olfactory receptors β subtype （OR-β） in the evolution of feeding adaptation in fish， we analyzed the OR-β genes from 12 fish species with different feeding habits including Megalobrama amblycephala， Ctenopharyngodon idellus， Cynoglossus semilaevis， Gadus morhua， Gasterosteus aculeatus， Larimichthys crocea， Paralichthys olivaceus， Danio rerio， Oreochromis niloticus， Oryzias latipes， Sinocyclocheilus graham and Xiphophorus maculatus. We also detected the expression patterns of the OR-β in the muscle， olfactory epithelium， olfactory bulb and brain of M. amblycephala. The results of evolutionary analysis showed that the number of OR-β varied greatly among fish species with different feeding habits. Herbivorous M. amblycephala and C. idellus had 20 copies and 14 copies， respectively， while carnivorous and omnivorous fish had an average of 1 and 4 copies， respectively. The results of selection pressure analysis showed that a total of 19 branches undergo positive selection， including 9 OR-β genes and 10 branch sites. Among them， the OR-β-4 and OR-β-14 gene of M. amblycephala （P<0.01） experienced strong positive selection. In addition， the OR-β of several Cyprinidae species， such as M. amblycephala， C. idellus， D. rerio and S. grahami， were clustered into one brand， which was positively selected and included 9 OR-β genes of M. amblycephala. The results of qPCR showed that 10 OR-β genes were highly expressed in the olfactory epithelium of M. amblycephala at 12-months of age and 24-months of age. OR-β-9 and OR-β-10 were also expressed in the olfactory bulb and brain tissue， while other OR-β genes were not expressed. Taken together， these results showed that the OR-β genes of the two herbivorous fish were specifically expanded and highly expressed in the olfactory epithelium， suggesting that OR-β might play potential crucial role in adapting the diets of herbivores.