Aim: To evaluate the effect of addition of Hydroxyl-Apatite nanoparticles to a commercial universal adhesive on sheer-bond-strength to dentin using two application modes, immediately and after thermocycling. Methods: A-commercial UA was used for bonding composite-resin to dentin and considered as control. 10% of HAp were prepared and added to the commercial UA and considered as experimental. 88-freshly extracted sound human premolar teeth were sectioned to expose midcoronal-dentin. Samples were y divided into: control and experimental group. Each group was further divided based on the application protocol. Half of the samples were stored in distilled water for 24h at 37°C for immediate SBS testing whereas the other half was subjected to thermocycling for 20,000 cycles. SBS was measured one-day post polymerization and after thermocycling using universal testing machine. SBS values and failure modes were recorded and statistically analyzed.Results: the experimental group presented significantly lower resin-dentin SBS than the control group. Thermocycling significantly lowered SBS values for both groups. When using SE mode, the experimental group reported lower resin-dentin SBS than the control group before and after thermocyclying. When using ER mode before thermocycling, no significant difference was found between the two groups, while after thermocycling, the experimental group reported significantly higher resin-dentin SBS than the control group. For both adhesive groups, mixed failure dominated with ER mode while adhesive failure dominated with SE mode.Conclusions: Addition of HA may provide resistance to degradation and offer a potential to improve resin-dentin bond stability against thermal changes when ER protocol is applied.