Spirulina platensis is a functional food duo to its bioactive compounds including phycocyanin, carotenoids, gama-linolenic acid and etc. However, the use of spirulina in food products affects their sensory characteristics and makes it unfavorable. One of the ways to overcome this problem is to extract the spirulina bioactive compounds, including phycocyanin. Yogurt containing the phycocyanin bioactive colorant, owe their consumer satisfaction to their potential health benefits. It was the objective of study to evaluate 1. The effects of pH and ionic strength of the buffer solution during the extraction step and the optimization of polyethylene glycol molecular weight as well as PEG and salt concentrations in the purification step to achieve the highest phycocyanin purity 2. The effects of phycocyanin on the antibacterial and physicochemical properties of yogurt and 3. Effect of yogurt containing different doses of phycocyanin on C57BL/6J mice obesity. For this purpose, phycocyanin was extracted by freeze-thawing and purified using an aqueous two-phase system before the freeze-dried phycocyanin was subjected to microbial analysis. To prepare phycocyanin-enriched yogurt, when yogurt pH reached 4.5, purified phycocyanin was added, mixed and stored at 4 °C. In Invivo studies, C57BL/6J mice were gavaged with low and high doses of phycocyanin-enriched yogurt for a month. mice weight changes, hematological factor and liver pathology was evaluated. Results showed that highest purity in the extraction step was obtained at pH 6.0 for all the buffer concentrations used. In the purification step, the optimal values for maximum phycocyanin purity (0.57), concentration (10.10 mg/mL), yield (134.21%), and purification factor (1.63) were determined using PEG 1000 g/mole, PEG 8.23% (W/W), and salt 23.41% (W/W), respectively. In yogurt, Streptococcus thermophilus and Lactobacillus bulgaricus counts decreased significantly on days 21 and 14, respectively, in all the treatments. Pathogens were detected in none of the treatments. The phycocyanin enriched-yogurts was found to have a higher pH than the plain control and they showed increased viscosity with increasing phycocyanin concentration. The control yogurt showed a higher syneresis, which increased by day 14 but decreased on day 21 due to increases in the hydrophilicity of yogurt proteins. This is while the b* index did not decline significantly by day 14. PE-yogurt 4% (W/W) was not significantly different in overall acceptability from the plain yogurt. In invivo study, high-fat diet had significant effect on the weight of mice than normal diet during 30 days. The liver weight to body weight was decreased in mice fed with low and high doses of phycocyanin and its ratio was similar to that of the control group. The hematological factors showed that the high fat diet, as compared with normal diet, will result in significant increases in factors such as fasting blood glucose, LDL cholesterol and liver enzymes. While, these effects was moderated in mice group fed with high fat diet and phycocyanin-enriched yogurt. The pathology of mice liver tissue at the end of 30-day period showed that high fat diet reduced the traarency of hepatocytes and causes cellular disorder. In addition, with the release of fat droplets, the border between cytoplasm and nucleus will be less colorful. But liver cells returned to normal in mice group fed with high-fat diet and phycocyanin-enriched yogurt. Finally, because of positive effects of phycocyanin on yogurt such as decreasing syneresis, increasing firmness, color stability, no pathogen growth and anti-obesity effects, it can be suggested that the use of phycoyanin as a bioactive compound in the production of functional yogurt. Keywords: phycocyanin, yogurt, color stability, obesity.