OBJECTIVE To evaluate the effect of a surface sealant agent on the color stability and surface roughness (Ra) of two denture base materials: polymethyl methacrylate (PMMA) and nylon. METHODS A total of 96 disk-shaped specimens (10 × 2 mm) were fabricated from heat-cured PMMA and nylon denture base materials (n = 48/material). Each of the color stability and Ra tests were performed on half of the samples in each material group (n = 24), and these subgroups were then subdivided according to the surface treatment applied: 12 specimens were conventionally polished, serving as the control group, and 12 specimens received a sealant on the surface in addition to the surface polishing. Color stability was assessed through measuring the CIEL*a*b* color parameters before and after immersion in coffee solution. Ra was measured and scanning electron microscopy (SEM) photomicrographs were taken before and after thermocycling. Statistical analysis was done using 2-way analysis of variance (α = .05). RESULTS The surface sealant agent significantly increased the color stability of both materials (P = .047). However, it had no significant effect on the Ra of materials before (P = .600) or after (P = .583) thermocycling. Nylon had a significantly rougher surface than PMMA both before and after thermocycling (P < .001). Similarly, SEM observations show a more irregular texture for nylon than for PMMA. CONCLUSIONS Although the sealant agent had no significant effect on the Ra before or after thermocycling, it significantly decreased the color change of nylon and PMMA disks. Based on both statistical analyses and SEM observations, the Ra of PMMA was lower than nylon denture base material.