The effect of Al addition on the interlamellar spacing of pearlite in ultra high carbon steels (UHCS) has been investigated. 0.32∼1.75wt%Al were added to 1.2%C-1.5% Cr-UHCS. Isothermal pearlitic transformation experiments were performed at undercoolings of 50℃, 100℃ and 130℃ below the eutectoid transformation temperature. When transformed at undercoolings of 100℃ and 130℃, the interlamellar spacing of pearlite decreased with Al contents. This fact resulted from the decrease of interfacial energy due to the reduction of lattice mismatch between ferrite and cementite by Al partitioning. However, at the undercoolling of 50℃, the interlamellar spacing was independent of Al contents due to dominant Cr partitioning.