Background and objectives: Tragacanth gum (GT) is one of the natural secreted hydrocolloids which has many applications in food and pharmaceutical industries. Unfortunately, there is limited scientific information about the electrostatic interactions of tragacanth gum with proteins, especially the proteins unfolded by physical approaches. In this regard, the main objective of this study was to investigate the effect of thermal processing on the whey protein isolate (WPI) properties and its COMPLEXation with tragacanth gum. Materials and methods: The influence of heating time (0, 5, 10, 15, 20, 25, 30 and 35 min at 80 C) on the turbidity of WPI, alone and mixed with GT (r = 0. 1, pH 4. 0 and Cp 0. 1 w/w) was surveyed. Then, the flow behaviour and VISCOSITY of GT-WPI mixture at a shear rate of 0-100 S-1 (pH 4. 0) was compared with the individual solutions of gum and protein. Since the maximum turbidity was observed in the range of 15-35° C, the impact of heating time at three levels (15, 25 and 35 min) and biopolymer mixing ratio at six levels (2: 1, 1: 1, 1: 2, 1: 5, 1: 10 and 1: 20) on the protein and carbohydrate compositions of precipitate and supernatant was investigated by Lowry and phenol-sulphuric acid methods, respectively. Results: Turbidity of protein solutions significantly changed with rising the heating time such that the maximum turbidity achieved after 25 min thermal processing. While these solutions were unstable due to protein-protein interaction and formation of protein aggregates, GT addition led to significant changes in the stability of protein solutions as a result of protein-polysaccharide interactions. Similar to protein solutions, the maximum turbidity of the COMPLEXes was achieved after 25min thermal processing. The rheological measurements showed that the mixture solution had pseudoplastic behavior with a large hysteresis loop as well as the higher VISCOSITY rather than the blank solutions, indicating the formation of electrostatic interactions between protein and polysaccharide. By determining protein and polysaccharide content in the supernatant and precipitate phases, the least ratio of Pr: PS was observed for the 25-min heated WPI. Furthermore, the ratio of Pr: PS in the precipitate was increased significantly as Pr: PSratio was increased in the mixture. Conclusion: Addition of tragacanth gum to the whey protein isolate led to a significant stability in the solution. Due to unfolding of WPI, using thermal processing on the whey protein isloate solution resulted in a meaningful increase in the turbidity of protein solution and the efficiency of GT-WPI COMPLEX formation.