A series of batch experiments were conducted to investigate the effects of inoculum type, oxygen (O-2) dosage, and incubation time on volatile fatty acids (VFAs) production during anaerobic digestion (AD) of Napier grass (Pennisetum purpureum), a high yielding energy crop. The results showed that anaerobically digested cattle manure (ADCM) as an inoculum generated significantly higher VFAs compared to that of anaerobically digested waste activated sludge (ADWAS) as an inoculum. Additionally, the incubation time of 3 days and O-2 dosage of 15 mL/g volatile solids(added) showed the highest VFAs production when ADCM was used as an inoculum. Moreover, the VFAs production had a quadratic correlation with O-2 dosage with R-2 of 0.86. The Scanning Electron Microscopy (SEM) images of the digested fiber showed rough and crumbled surface structures as opposed to that of the undigested fiber, which was further confirmed by changes in structural composition of the digested fiber. (C) 2017 Elsevier Ltd. All rights reserved.