Background: The oral cavity is a challenging environment that promotes aging of dental restorations. Aging can lead to surface roughness causing bacterial adherence. Therefore, surface roughness and bacterial colonization of recently introduced gradient zirconia upon aging are under investigation. Aim: to assess the surface roughness and bacterial colonization of two types of zirconia (cubic and gradient) after accelerated hydrothermal aging. Materials and methods: Twenty-six disc-shaped samples (12mm X 1.2mm) were CAD/CAM fabricated. Samples were divided into two equal groups (n=13) according to the material type: Group B: cubic zirconia (BruxZir)and Group I: gradient zirconia (IPS e.max ZirCAD Prime). Finishing, polishing, and glazing of the samples were done followed by surface roughness measurement for each disc using a 3D non-contact profilometer. This was followed by incubating the samples in artificial saliva and bacterial suspension then total bacterial count of each disc was assessed and calculated using colony-forming units (CFUs/ml). Results: Regardless of aging IPS e.max ZirCAD Prime (0.2535± 0.0043) showed lower surface roughness than BruxZir (0.2552 ± 0.0037). Both types of zirconia showed an increase in surface roughness after aging (0.2581± 0.0017). Both types of zirconia; BruxZir (5.22 ± 0.81) and IPS e.max ZirCAD Prime (5.18 ± 0.57) showed an equal amount of bacterial colonization. Both types of zirconia showed an increase in bacterial colonization after aging (5.73, ± 0.46). Conclusion: Surface roughness of both materials increased due to aging. Both materials showed an increase in bacterial counts after aging. There is a positive correlation between surface roughness and bacterial colonization.