During heat treatment, whey proteins (WP) are denatured and form different kinds of whey protein aggregates (WPA) depending on the physico-chemical conditions. In the presence of casein micelles, mixed aggregates (MA), resulting from the interaction of k-casein and WP, are formed. The aim of this study is to understand the effect of pH on the WPA structure and the impact of the casein/whey proteins (Cas/WP) ratio on MA morphology. In this work, WP solutions were heated at pH 5.8 or 7 to produce WPA. On the other hand, aqueous mixtures containing three Cas/WP ratios (82/18, 56/44 and 30/70) were heated at 80 C during 1 h at pH 6.3 to obtain MA. Milk protein aggregates were analyzed by Transmission Electron Microscopy (TEM) and Asymmetrical Flow Field-Flow Fractionation coupled with Multiangle Laser Light Scattering and Differential Refractometer (AF4-MALLS-DRi). Dense and spherical WPA were formed at pH 5.8 whereas branched and fractal aggregates were obtained at pH 7. Regarding MA, results suggested that they were mainly produced with the 82/18 Cas/WP ratio whereas a majority of WPA was obtained with the 30/70 ratio. At intermediate ratio, the mixture was composed of small MA and WPA that did not interact with k-casein. Moreover, WP seem to interact preferentially with larger casein micelles. Thereby, AF4-MALLS-DRi proved to be a powerfull technique to characterize the complex structure of milk protein aggregates and an interesting alternative to size exclusion chromatography especially for MA and casein micelles which interact with the stationary phase and are retained in the column