Abstract Statement of the Problem: Composite bond to dentin is crucial in many clinical conditions particularly in deep cavities without enamel margins due to insufficient penetration of adhesive into demineralized dentin.Purpose: The aim of this study was to assess the shear bond strength (SBS) of a methacrylate-based and a silorane-based composite resin to surface and deep dentin after pretreatment with dimethyl sulfoxide (DMSO).Materials and Method: Eighty extracted human premolars were randomly divided into two groups of flat occlusal dentin with different cuts as A: surface group (sections just below the dentinoenamel junction (DEJ) and B: deep group (2 mm below DEJ). Each group was randomly assigned to 4 subgroups and their samples were restored with Adper Single bond (ASB) and Filtek Z350 or Silorane system Adhesive (SA) and Filtek P90 composite resins, using a 3×3mm cylindrical plastic mold. following these steps , the subgroups were assigned as SubgroupA1: surface dentin+ Silorane System Primer (SSP)+ Silorane System Bonding (SSB)+ P90; Subgroup A2: surface dentin+ 37% etchant (E37%) + Adper Single Bond (ASB)+ Z350; Subgroup A3: surface dentin+ DMSO+ SSP+ SSB+ P90; Subgroup A4: surface dentin+ E37%+ DMSO+ ASB+ Z350; Subgroup B1: deep dentin+ SSP+ SSB+ P90; Subgroup B2: deep dentin+ E37%+ ASB+ Z350; Subgroup B3: deep dentin+ DMSO+ SSP+ SSB+ P90; Subgroup B4:dentin +E37% +DMSO +ASB +Z350. The specimens were thermocycled at 5± 2/55± 2°C for 1000 cycles and then tested for SBS.Results: Using DMSO as dentin conditioner increased SBS of ASB to deep dentin (p< 0.001) and SBS of SA to surface dentin (p= 0.003) but had no effect on SBS of SA to deep dentin (p= 1.00).Conclusion: The ability of DMSO to increase SBS of ASB to deep dentin provides a basis for improving bonding of this composite resin in deep cavities. Keywords ● Dentin pretreatment ● Dimethyl sulfoxide ● Bond strength, deep dentin ● Surface dentin ● Etch and rinse adhesive ● Self-etch adhesive