Nowadays, essential oils have important roles in human health and life. They are used in pharmaceutical, food, and cosmetics industry. Peppermint (Mentha piperita L.) belongs to the family Lamiaceae, containing essential oil and is one of the important medicinal plants. Because of it’s important in mentioned industries an investigation was done in order to survey the effect of seaweed liquid fertilizer on peppermint essential oil. The treatments were four levels of seaweed liquid fertilizer (0, 1/5, 2, 2/5 cm3/liter). The experiment was carried out in Khoy, Iran. Traits were measured including oil yields (v-w), menthol content and essential oil content. According to the analysis of variance result seaweed liquid fertilizer treatments had no significant effect on essential oil content, volumetric efficiency oil and weight efficiency oil and menthol content. However, seaweed application had no significant effects on different studied traits of peppermint essential oil there observed a difference between seaweed liquid fertilizer application treatments and control so essential oil content of the seaweed liquid fertilizer treated plants were higher than control. According to the results, we suggest seaweed liquid fertilizer application as supplementary with other kind of fertilizers in peppermint.