Corn stover and switchgrass are two important feedstocks considered for producing renewable fuels and energy in the U.S. Densification of these biomass materials into briquettes or pellets would help reduce the problems and costs of transportation, handling, and storage. The objective of this study was to investigate the binding mechanisms of corn stover and switchgrass in briquettes and pellets. Scanning Electron Microscopy (SEM) images of briquettes made using a roll press briquetting machine, and pellets made using a conventional ring-die pelleting machine were analyzed. The SEM images showed that the bonding between particles was created mainly through solid bridges. The solid bridges between the particles were made by natural binders in the biomass expressed during the densification process. UV auto-fluorescent images of briquettes and pellets further confirmed that the solid bridges were made mainly by the natural binders such as lignin and protein. It was found that activating (softening) the natural binders using moisture and temperature in the range of glass transition is important to make durable particle-particle bonding.