The yield stress of concentrated wheat straw suspensions was measured by a vane with a Bohlin rheometer. Wheat straw fibers were made by grinding (wet and dry) wheat straw and separated into four sizes by sieving (8, 12, 20, and 40 mesh). The yield stress was found to increase with concentration as well as with the size of the wheat straw fibers. The yield stress data fitted to power law equation proposed by Kerekes et al. (1985) for pulp fibers. It was found that only 40 mesh size wheat straw fibers agreed with the relation. Effect of stress ramp rate was studied on 5% (20 mesh) w/v, wheat straw suspension. It was observed that stress ramp rate had an effect on measured yield stress value. No yield stress was observed in the suspension of 5% (40 mesh) w/v, due to sedimentation of wheat straw fibers. For effective utilization and mixing of enzymes with wheat straw fibers during hydrolysis, knowledge of yield stress of suspension was important.