Cellulosic ethanol made from cellulosic biomass is an alternative to petroleum-based transportation fuel. The cost-effectiveness of cellulosic ethanol manufacturing has been hindered by several technical barriers. One such barrier is that low density of biomass causes high costs of biomass transportation, handling, and storage. Another barrier is low sugar yield in enzymatic hydrolysis, making enzymatic hydrolysis an expensive and slow step. Ultrasonic vibration-assisted (UV-A) pelleting of cellulosic biomass can increase its density and reduce the costs of biomass transportation and handling. The effects of UV-A pelleting on sugar yield of cellulosic biomass have not been fully investigated. The objective of this paper is to investigate effects of UV-A pelleting on chemical composition and sugar yield of cellulosic biomass. The effects were investigated with and without dilute acid pretreatment using corn stover and sorghum stalk. It was found that there was no significant difference in chemical composition between pelleted and unpelleted biomass whether they went through dilute acid pretreatment or not. After dilute acid pretreatment, cellulose recovery of pelleted biomass was significantly higher than that of unpelleted biomass. UV-A pelleting could significantly increase the sugar yield in enzymatic hydrolysis for both corn stover and sorghum stalk. (C) 2014 Elsevier Ltd. All rights reserved.