Wheat straw is a good source for the production of bioethanol. It can be converted into smaller fibers using mechanical treatment such as milling and grinding. These fibers can then be suspended in water and the slurry behaves as a non-Newtonian fluid possessing yield stress. In mixing operations, the presence of yield stress creates a region of active motion (called cavern) around the impeller, and stagnant zones in the rest of the tank. 2D and 3D electrical resistance tomography (ERT) images of wheat straw slurries were used in this study to measure the cavern diameter and height, respectively, created by mixing the slurries, and to estimate their yield stress from these dimensions. The average yield stresses of 5, 7, and 10 wt% slurries were 1.31 Pa, 4.2 Pa, and 14.8 Pa, respectively, when fiber size was ≤ 2 mm, and 3.4 Pa, 6.8 Pa, and 16.7 Pa, respectively, when fiber size was 8 mm ± 0.014 mm. The author believes that this study is the first novel application of ERT to estimate the yield stress of wheat straw slurries, as opposed to directly measuring it with a rheological instrument.