Abstract: Post-acidification of set yoghurt always occurs during cold storage causing short shelf life thus consumer unacceptability. The short storage life of particular yoghurt affects the consumption level as well. This study concentrated on the inhibitory effect of preservatives on post-acidification of yoghurt. The set yogurt mixture was produced and inoculated with 2% starter culture containing Streptococcus thermophilus and Lactobacillus bulgaricus. After the required inoculation; ε-polylysine (as natural preservative) and potassium sorbate (as chemical preservative) were added separately at different concentrations. Titratble acidity, pH and firmness of yogurts during cold storage for 20 days were examined. Evaluation of sensory properties was done for the best concentrations of each preservative. The pH values and titratable acidity were significantly lower (<0.05) in yoghurts with added preservatives than those in blank yoghurt. The results showed that ε-polylysine and potassium sorbate used at 0.005% (w/v) and 0.1% (w/v), respectively gave the best (optimum) inhibition of post-acidification during refrigerated storage for 20 days. The sensory evaluation revealed that 0.005% (w/v) ε-polylysine was the most preferred preservative to consumers with respect to taste, colour, mouth feel, appearance and overall acceptability. Hence, these results support the conclusion that there is an inhibitory effect of potassium sorbate and ε-polylysine on post-acidification of set yoghurt during cold storage while the later was most preferred by the panelists.