Plastic instability and strain rate sensitivity (SRS) of the ultrafine-grained (UFG) iron subjected to asymmetric rolling (ASR) have been studied through uniaxial tensile test. In addition to the high strength due to grain refinement, the tensile curve of the UFG iron shows severe plastic instability. The plastic instability becomes more with the decreasing SRS value. The low SRS value results in the severe plastic instability. The SRS value within the grain size range from 0.9 to 25μm has been measured and is found to decrease with the decreasing grain size. The evolution of the SRS value against the grain size is explained using a simple model based on H–P relationship.