Cosmetics are often contaminated by pathogenic bacteria, such as Staphylococcus spp. and Pseudomonas spp. During manufacturing, preservatives are added to cosmetic products to protect them from microbial contamination. In this study, the effects of four preservatives, phenoxyethanol (PE), benzyl alcohol (BA), propylparaben (PP), and methylparaben (MP), against seven microbial strains isolated from contaminated cosmetics were examined. The isolated bacterial strains included Bacillus cereus, Bacillus subtilis, Escherichia coli, and Staphylococcus aureus and the isolated fungal strains were Aspergillus flavus, Aspergillus fumigatus, and Aspergillus welwitschiae. Bacterial strains were identified using 16S rRNA sequencing, whereas the fungal strains were identified using 18S rRNA sequencing. The minimum inhibitory concentrations (MICs) were calculated to determine the sensitivity of the strains to the four preservatives (PE, BA, PP, and MP) at different concentrations (3.2, 1.6, 0.8, 0.4, 0.2, 0.1, and 0.05 mg/mL). The results showed that most of the bacterial strains were resistant to PP, and their growth was inhibited at high concentrations (0.8-3.2 mg/mL) of each preservative. B. subtilis was the most sensitive bacterial strain to preservatives, at low concentrations (0.1-0.8 mg /mL). The MICs of MP ranged from 0.05-0.2 mg/mL, which indicates the strength of their inhibitory effect on fungi at low concentrations. Growth of microorganisms inside cosmetic products is an issue that needs to be reconsidered by researchers and preservative systems should be evaluated. For safety and health purposes of customers usually use cosmetics, this study is prepared.