In this study, the efficiency of the plant age and growth period on antioxidant and antimicrobial activity of stevia (Stevia rebaudiana Bert.) was investigated. Samples were taken from S. rebaudiana plants of three different ages (2, 3 and 4 years old) and four different growth periods (H1: 1 July, H2: 1 August, H3: 1 September and H4: 1 October). Extracts of sweet herb (S. rebaudiana) were obtained by maceration method with methanol as solvent. The experiments were carried out in completely randomized design with three replications. According to results, the highest antioxidant activity (metal chelating activity of methanol extract from stevia leaf; DPPH Free Radical Scavenging Activity of methanol extract from stevia leaves) was determined from respectively No.2 sample (3 year-old plant harvested in July) and No.1 sample (2-year-old plant harvested in July). The highest antimicrobial and antifungal activity for Escherichia coli and Staphylococcus aureus was determined from No.9, No.10, and No.11 samples (in the order of 3-year-old plant harvested in September, 3-year old plant harvested in October, and 4-year-old plant harvested in November). All of the stevia samples harvested in different age and different periods were showing antimicrobial effect against Pseudomonas aeruginosa, E. faecalis and Candida albicans.