This study examined the change in the dyeability of natural dyes on cotton fabrics by a PEI (polyethyleneimine) pretreatment instead of mordants. Cotton fabrics were treated with PEI and the changes in the dyeability were shown by measuring the amount of PEI on cotton fabrics. Samples treated with PEI were dyed with two natural polychromatic dyes with a different affinity to cotton fibers: Alizarin Red S and Curcumin. The changes in dyeability by three variables (time, temperature and concentration of dyes) on cotton fabrics were analyzed by the K/S value to define optimum dyeing conditions. Subsequently, the PEI treatment improved the dyeability of cotton fabrics with both dyes of low and high affinity to cotton fibers. Thus, PEI could be a suitable heavy metal mordant replacement.