The effects of heat treatment and whey protein addition on the fermentation time and the rheological properties of kefir determined using a pneumatic tube viscometer of novel design, were investigated. Kefir samples were prepared with and without the addition of whey protein concentrates (WPC) at varying concentrations, while heat treatment of the milk was made prior or after WPC addition (resulted to native or denatured whey proteins, respectively). Increasing WPC concentration resulted to increased lactic acid concentration and reduced fermentation time. The flow curves of the samples suggested that kefir behaves as a pseudoplastic fluid. The apparent viscosity of kefir samples was increased with increasing WPC concentration. However, the “state” of added whey proteins greatly affected the rheological properties of kefir. Particularly, denatured whey proteins resulted to higher consistency index values and thus apparent viscosity and lower flow behavior index values, when compared with native whey proteins. Practical Applications Kefir is a fermented beverage, which is reported of being beneficial to health. The large number of different microorganisms found in it and the variety of bioactive compounds produced during fermentation distinguishes it from other probiotic products. The study of the rheological properties of kefir as well as the factors that have an effect on them is of great scientific and commercial interest, because they can affect the quality of the final product and thus its acceptance by the consumers, the promotion of its standardization, and allow the correct selection of equipments like pumps and filling machines in the industry. The addition of whey protein concentrates (WPC) can enhance rheological properties of kefir and thus improve the quality of the final product. Furthermore, the “state” of the added whey proteins greatly affects the impact degree on the rheological behavior of kefir.