In ductile cast irons with copper, cementite stability was investigated against an annealing heat treatment used to obtain a fully ferritic matrix. Copper controls cast-iron mechanical properties, but its role in the matrix microstructure formation remains unclear. Some reports suggest the copper layer around graphite or cementite. They can be barrier to carbon diffusion at eutectoid reaction, however it is difficult to understand the mechanism of pearlite stability by copper. To confirm the existence of the barrier and effect of copper addtion, ten 9-mm-thick spheroidal graphite cast iron castings were prepared with different copper contents of 0.16 wt% – 0.69 wt%. The samples’ as-cast microstructures included spheroidal graphite, ledeburite, and pearlite. The pearlite fraction degreases to about 10% by heat treatment for ordinary ductile irons without intentional copper addition. The samples’ copper content and the pearlite fraction after heat treatment are not linearly related. The retained pearlite increased suddenly with increased copper content greater than 0.4 wt%. However, even the sample with the highest copper content showed no precipitation of a copper solid solution around graphite nodule or cementite.