We investigated how air thinning of the bonding resin affected the bond strength to enamel and dentin of the three self-etching primer adhesive systems, Clearfil Megabond, UniFil Bond and Imperva Fluorobond, by measuring the tensile bond strength to bovine enamel and dentin. There was no significant difference in the bond strength to enamel between brush thinning and air thinning for the three adhesive systems. However, the air thinning significantly reduced the bond strength to dentin for the three systems (p<0.01). We concluded that thinning of the bonding resin with a brush achieved a favorable bond strength to dentin compared with air thinning.