Biofuels produced from cellulosic biomass are an alternative to petroleum-based liquid transportation fuels. The costs of cellulosic biofuel manufacturing are high partly due to the low density of raw cellulosic materials. Processing cellulosic biomass into pellets can increase density and handling efficiency of cellulosic feedstocks, resulting in reduction of transportation and handling costs. The literature on ultrasonic vibration-assisted (UV-A) pelleting includes studies about effects of pelleting parameters on pellet quality, pelleting temperature, charring, and sugar yield. However, there are no studies on pellet cracks. This paper, for the first time, presents a study on pellet cracks in UV-A pelleting of wheat straw. It first describes experimental conditions and then presents experimental results about effects of pelleting parameters on crack size. Afterward, it proposes a hypothesis on crack formation in UV-A pelleting and discusses some evidences that support this hypothesis. Finally, it provides explanations to experimental results based on the hypothesis.