The cotton mealybug, Phenacoccus solenopsis Tinsley, is a polyphagous pest with a wide range of host plants which has caused several damages to ornamental plants in some regions of Iran. In this study, the contact toxicity of five essential oils from Lamiaceae family including Mentha viridis L., Mentha pulegium L., Satureja horthensis L., Zataria multiflora Bioss, and Rosemarinus officinalis L. was investigated against the third instar nymphs of cotton mealybug. Essential oils with concentrations of 4.5, 9, 13.5,18 and 22 µL/mL of acetone were used in these experiments. The leaves of Hibiscus rosa-sinensis L. were placed in essential oil solutions for 10 seconds and then dried in air for five minutes. The surface of Petri dishes contained agar were covered with the leaves which were exposed to different concentrations of essential oils and ten 3rd instar nymphs of mealybug were released on the leaves. Four replications were considered for each concentration. Acetone was used as control. Then the lids of Petri dishes were closed and they were transferred to an incubator with the temperature of 27±1˚C, 65±5% R.H. and 12:12 L: D to evaluate insects’ mortality. Data obtained from dose-response bioassays were subjected to probit analysis to estimate LC50 values using SAS software. The results showed that all the essential oils have insecticidal activity against P. solenopsis at all the tested concentrations. The calculated LC50 values were 8.1, 12.5, 14.2, 18.2, and 19.9 µL/mL of acetone for M. pulegium, M. viridis, S. hortensis, Z. multiflora, and R. officinalis, respectively. According to these results, the essential oils of M. pulegium and R. officinalis had the most and the least effects on the mortality of mealybugs, respectively. This study demonstrated that all the tested essential oils have the ability to control P. solenopsis nymphs and could be useful in the development of new agents for mite control.