Objective: to assess micro-shear bond strength (μSBS) of two universal adhesives (UA) having two etching aggressiveness applied to dentin in a self-etch mode, immediately and after thermal ageing. Methods: the occlusal enamel of twenty-eight sound human premolar teeth were removed to expose a flat dentin surface for bonding procedures. Teeth were randomly assigned to one of two groups according to the adhesive; the first group (Peak-Universal: PU) has intermediate etching aggressiveness (1.2 pH) and the second group (All-Bond-Universal: ABU) has ultra-mild etching aggressiveness (3.2 pH). After bonding procedures as per manufacturer instructions, a nanohybrid composite resin was placed and light cured. All specimens were stored in demineralized water at 37˚C for 24 hours. Half of the specimens were evaluated for μSBS testing after 24 hours of water storage at 37˚ C and the other half were evaluated after thermal ageing using10000 thermal-cycles and six-months water storage, then observed by SEM. μSBS in (MPa) were compared with analysis of variance. Kolmogorov-Smirnov and Shapiro-Wilk tests were applied to assess normality of distributions (P < 0.05 was considered as significant). Results: PU showed statistically significant higher mean μSBS than AU, immediately and after thermal ageing. Thermal ageing and water storage did not significantly influence PU while bond degradation was significant in AU. Conclusion: with regard to the adhesive systems used in this study, when self-etch mode was used, UA with aggressive pH (intermediate) revealed better bonding effectiveness and stability against thermal aging and water storage than those with ultra-mild pH.