This study was conducted to investigate the effect of acidic and/ or mechanical challenge on the surface roughness of flowable resincomposites. Ninety six resin composite discs were prepared according tomanufactures instructions. The specimens were divided into two maingroups of 48 specimens each, according to the type of flowable resincomposite used; nanohybrid flowable resin composite Tetric N Flow(F1), or microfilled flowable resin composite Heliomolar Flow (F2). Eachgroup was further subdivided into four equal subgroups of 12 specimenseach according to the type of challenge. First: the control groupspecimens were stored in artificial saliva at pH 7.4 (C0). In the secondgroup the specimens were subjected to an acidic challenge with orangejuice at pH 5 (C1). For the third group the specimens were subjected tomechanical abrasion challenge by automatic tooth brushing (C2). Finally,the specimens in forth group were subjected to an acidic challengefollowed by mechanical abrasion challenge (C3). Specimens of eachgroup were examined for surface roughness at base line R0 and afterexposure to the proposed challenge R1. The change in the surfaceroughness values following application of different challenges weretested using USB Digital microscope with a built-in camera, using fixedmagnification of 120X. Results were collected, tabulated and statisticallyanalyzed.The collected results showed that, no statistical significantdifference was found between the tested flowable resin composites orbetween the different applied challenges and the control group. Regardingthe results of the surface roughness at the base line and after the appliedchallenges a statistical significant decrease in the surface roughness wasdetected. The nanohybrid resin composite group which received themechanical challenge showed the lowest mean surface roughness.However, it was not statistically significant than all other groups. TheSummary and conclusions67nanohybrid resin composite control group showed the highest meansurface roughness, which was also not statistically significant than allother groups.Within the limitations of the present investigation, it could beconcluded that:1) The surface roughness of the tested resin composites were notinfluenced by the type of the material as both types performedsimilarly under the different challenges types.2) The acid used in this study was able to change the surfaceroughness of the tested materials which indicate that the patient'sfruit beverage consumption habit could affect the longevity of theresin restorations.3) The two tested resin composites presented similar surfaceroughness after simulated tooth brushing4) No synergistic effect was evident after the combination betweenacidic and mechanical abrasion challenges.5) Exposure to acidic and mechanical challenges altered the surfaceroughness of both evaluated flowable resin composite.