Stress-strain curves of iron whiskers, of which thickness ranges from 5 to 80μ, have been measured using an Instron-type tensile machine at room temperature. All of the whiskers show "work softening" of which softening rate decreases with increasing thickness. "The lower yield stress" decrease with increasing thickness in and whiskers thinner than about 35μ. Marked thickness and orientation dependence of the stress-strain curve have been found. The whiskers thinner than about 8μ show no appreciable plastic deformation after the sharp yielding, as already known in very thin whiskers. Those, 8～17μ in thickness, show nearly flat regions in their stress-strain curves after yielding and those thicker than about 17μ reveal "work hardening" after about 5% strain
