Objective This study was to find out the most suitable concentration of added NaF to enough bonding strength of brackets.Methods Sixty-four teeth were randomly divided into A, B, C and the control group. NaF of four different concentra- tions (1·23%, 2%, 3% and 0%) was added into 35% phosphoric acid. The fourgroup teethwere etchedwith four kinds of flu- oride etching fluid respectively andwere bondedwith brackets. Four specimens in each groupwere selected byturns after1, 4, 12 and 24 weeks of bonding. The shear bond strength of the teethwere tested, and the adhesive residual index (ARI) of bonding in- terface were recorded.Results The shear bond strength at fourtesttime in each groupwas no significant (P>0·05). Therewas statistical difference of the shear bond strength between fourgroups(P
