Abstract: Antimicrobial preservatives are commonly used in cosmetic products in order to reduce their microbial contamination. The effective concentration of these products is one of the most critical points in formulation of cosmetics. There are some standard test methods for determining the efficiency of antimicrobial preservatives in cosmetics and even in other products such as wood. No standard test method was found for determining the effectiveness of antimicrobials before adding to the final product in our searches. Minimum Inhibitory Concentration (MIC) method described for antimicrobial drugs was considered as a supportive tool for establishing a practical and proper method for preservatives. We assessed the differences between antimicrobials which are used as drugs or preservative and made some modification in the basic method. The modified method was use by 3 laboratories on the same samples and results with slightest differences showed that it might be practical as a standard test method. Further studies on reproducibility and repeatability are to be investigated.