Statement of the Problem: Evaluation of the color stability of materials used for the fabrication of the base of complete and partial removable dentures provides important information in relation to their clinical service. The color stability of nylon-based polymers as denture bases, compared to that of acrylic resins, has not been extensively evaluated since to date.Purpose: The aim of the present in vitro study was to evaluate the color stability of conventional polymethyl methacrylate and nylon-based polymer materials used for the fabrication of the base of complete and partial removable dentures.Materials and Method: Twenty samples were evaluated in the present in vitro study. One nylon-based polymer and one heat-cured acrylic resin were seleted for the purpose of the study. Ten disk samples, measuring 10 mm in diameter and 2 mm in thickness, were prepared from each material according to the manufacturers’ instructions. The samples were evaluted with the CIELAB sysem using spectrophtometery at baseline and after irradiation with UV light for 100 and 200 hours and thermocycling. One-way ANOVA and post hoc paired t-test were used to compare the color stability of the two groups. Statistical significance was set at p< 0.05.Results: After 100 hours of irradiation with UV light and thermocycling, color changes were tangible in both materials. After 200 hours of irradiation with UV light and thermocycling, both materials exhibited a decrease in glaze. In general, chromatic changes in the acrylic resin were more noticeable.Conclusion: Accelerated aging affected the color stability of the acrylic resin and nylon-based polymer, p< 0.05 for Δa200 and Δb200 and ΔL100; however, at the end of the study both materials exhibited clinically acceptable color stability (ΔE<3.7). Keywords ● Color stability  ● Spectrophotometer  ● Nylon-based polymer