Small amount of aluminum addition to ultrahigh carbon steels (UHCS) leads to finer grain size during the established thermomechanical processing, and is known to improve the superplasticity of the steels. However, the effect of Al on the grain refinement in the hyper-eutectoid steels during thermomechanical processing is not well recognized and has never been quantitatively documented. Up to 5wt% aluminum was added to 1.2∼1.5wt%C steels and its influence on microstructural refinement has been studied. That is, the formation of proeutectoid carbide, austenite grain size, pearlite colony size, interlamellar spacing, equiaxed ferrite grain size and carbide particle sizes as a function of the Al-content were determined after hot-warm working plus an isothermal rolling or the divorced eutectoid transformation with associated deformation. Fine grain microstructure consisting of less than 1㎛ ferrite grains has been achieved by small Al-addition. It has been demonstrated that the dilute Al-alloying enhances the superplasticity of the UHCS: the optimum strain-rate has shifted to the faster rate region(at 800℃) and the tensile elongation of 545% has been observed at an initial strain-rate of 8.3×10^(-2)sec^(-1).