The functionalization of styrene-b-(ethylene-co-1-butene)-b-styrene tri-block copolymer with maleic anhydride (MAH) in melt state through ultrasound initiation was studied in this article. The effects of plasticizer content and types, MAH content, ultrasound power, and die temperature on grafting ratio of MAH were investigated by means of acid-base titration. Functionalized products were confirmed by new absorption bands in Fourier-transform infrared spectroscopy. The experimental results showed that the ultrasound initiated products had lower complex viscosity (*), lower gel content as well as lower molecular weight than peroxide initiated products, indicating that the ultrasound could cause chain scission and suppress the crosslinking side reaction to gain functionalized products, which have less gel content and high grafting ratio. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013