This work was performed to study the effect of the covalent modifications of whey protein isolate (WP) by chlorogenic acid (CA) and rosmarinic acid (RA) on their physiochemical and emulsifying properties. Phenolic compounds, RA and CA, were covalently bound to WP as investigated by the decrease in free amino and thiol groups. Zetasizer apparatus and ANS (1-anilino-8-naphthalensulfonate) analyses showed some changes between modified and unmodified WP in zeta potential, particle size, isoelectric point, and surface hydrophobicity. Modified WP by CA had the increased surface hydrophobicity and isoelectric point, while modified WP by RA had the decreased isoelectric point compared with unmodified one. WP modified by RA and CA showed a change in emulsifying properties. Particle size and zeta potential of sunflower emulsion emulsified with 0.3% WP with and without modification were investigated using static light scattering (SLS). The results showed that WP modified by RA produced an emulsion with smaller particle size, while WP modified by CA showed the opposite. Moreover, both modified proteins formed emulsions with zeta potential higher than unmodified one. Finally, WP modified by RA improved the stability of emulsion against creaming compared with WP control and that modified by CA. Although the covalent interactions of the phenolic compound with whey protein isolate may decrease their nutritional properties, the phenolics – whey protein isolate conjugates are being considered as a way to develop functional foods due to their good emulsifying properties.