Two essential oils, namely thyme and clove, were added to plain yoghurt at concentrations of 0.2, 0.6 and 1.2 parts per million (ppm). Subsequently, the chemical, microbiological and organoleptic properties of freshly prepared yoghurt and of the yoghurt stored at refrigerating temperature for up to 21 days were determined. Addition of essential oils affected the acidity and total volatile fatty acid values of the prepared yoghurt. On the other hand, total solids were only slightly affected. Total bacterial counts, as well as counts of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus in the treated yoghurt increased and reached a maximum at 7th day of storage where after it decreased until the end of the storage period. Yeasts and moulds, coliform bacteria and spore-forming bacteria were not detected in the treated yoghurt. Of the different treated yoghurt, yoghurt containing 0.2 ppm thyme or clove oils was organoleptically the most acceptable, and it had a good body and texture that was similar to that of the untreated control. From the results of this study, it can be concluded that 0.2 ppm of thyme or clove can be used in order to increase the shelf-life of yoghurt for up to 21days.