The aim of this study was to examine the effects of seaweed extract (Algaren), nettle extract and microbial biostimulant (Slavol) on total phenolics and flavonoids and antioxidant capacity of leaves of lavender (Lavandula officinalis L.), sage (Salvia officinalis L.) and basil (Ocimum basilicum L.). The study was carried out in a greenhouse located in Vitez, Bosnia and Herzegovina. The experimental trial was set up in a randomized block design with four biostimulant treatments in three replications. The first application of biostimulant was carried out immediately after the transplanting of seedlings, and the second and third treatments 15 and 30 days later, respectively. Total phenolic and flavonoid contents and ferric-reducing/ antioxidant power (FRAP) assay were used to evaluate the antioxidant properties of each leaf extract. The nettle extract was found to be the most effective in increasing total phenolics, flavonoids, and antioxidant capacity in leaves of studied plants. In this study, the antioxidant capacity of leaf extracts was in line with total phenolics/flavonoids, regardless of plant species and biostimulant treatments, suggesting that phenolic compounds greatly contribute to antioxidant capacity of studied plants.