Wheat straw is a low-cost feedstock for production of biofuels as a viable alternative to fossil -based fuels. Pretreatment process is an important stage in producing biofuels. Pretreated wheat straw slurries (PWS) are non-Newtonian fluids with yield stress. In mixing operations, the presence of yield stress creates a region of active motion (cavern) around the impeller and stagnant zones elsewhere which causes difficulties in the production of biofuels. In this study, for the first time electrical resistance tomography (ERT) was utilized to measure the cavern dimensions as a function of the impeller type (A200, A100, and A315), impeller speed (20 to 110 rpm), fiber size (≤ 2 and ≤ 6 mm), and PWS concentration (6, 8, and 10 wt%). The cavern sizes were used to measure the yield stress of PWS slurries as a function of fiber size and fiber concentration. The average yield stresses of 6, 8, and 10 wt% PWS slurries with the fiber sizes of ≤ 2 mm were 2.00, 5.43, and 8.51 Pa, respectively, and 4.26, 9.30, and 13.84 Pa for the fiber sizes of ≤ 6 mm.