•ARG inhibited amyloid fibril formation by RCMκ-CN in vitro.•The enhancement in RLS intensity was attributed to the formation of RCMκ-CN–ARG complex.•ARG quenched the fluorescence of RCMκ-CN and the probable quenching mechanism was a static-quenching process.•ARG bound strongly to RCMκ-CN via intermolecular hydrogen bond and van der Waals forces.•The distance between ARG and RCMκ-CN Trp97 residue evaluated by FRET was 2.94 nm.Herein, the interaction of l-arginine (ARG) with κ-casein, and its effect on amyloid fibril formation of the protein, have been investigated in vitro by resonance light scattering (RLS), fluorescence, UV–Vis absorption spectroscopy and transmission electron microscopy (TEM) under simulated physiological conditions. The results indicated that ARG inhibited fibril formation by reduced and carboxymethylated κ-casein (RCMκ-CN), and there was interaction between ARG and RCMκ-CN, proved by the observation of enhancement in RLS intensity attributed to the formation of RCMκ-CN–ARG complex. It was also demonstrated that ARG strongly quenched the intrinsic fluorescence of RCMκ-CN through a static quenching mechanism. The corresponding thermodynamic parameters (ΔH, ΔS and ΔG) were tested to show that the binding process was spontaneous and mainly enthalpy driven with an unfavorable entropy, and both hydrogen bond and van der Waals forces played a key role in the binding of ARG and RCMκ-CN. The determined value of the distance r between ARG and RCMκ-CN Trp97 residue evaluated by fluorescence resonance energy transfer (FRET) was 2.94 nm. Furthermore, the conformational investigation from synchronous fluorescence showed that the RCMκ-CN Trp97 residue was placed in a less polar environment and more difficultly exposed to the solvent after addition of ARG.