The increasing status of insecticide resistant mosquitoes in sub-Saharan Africa is a threatening alert to the existing control efforts. All sibling species of An. gambiae complex have evolved insecticide resistance in wild populations for different approved classes of the insecticides currently in use in the field. An alternative compound for vector control is absolutely urgently needed. In this study, the larvicidal activity and chemical composition of the Cinnamomum osmophloeum leaf essential oils were investigated. C. osmophloeum leaf essential oils were extracted by hydrodistillation in a Clevenger-type apparatus for 6 hours, and their chemical compositions identified using GC-MS. These oils were evaluated against An. gambiae s.s. in both laboratory and semi-field situations. The WHO test procedures for monitoring larvicidal efficacy in malaria vectors were used. The composition of C. osmophloeum leaf essential oil has been found to have 11 active compounds. The most abundant compound was trans-cinnamaldehyde (70.20%) and the least abundant was caryophyllene oxide (0.08%). The larvicidal activity was found to be dosage and time dependant both in laboratory and semi-field environments with mortality ranging from 0% to 100%. The LC50 value was found to vary from 22.18 to 58.15 μg/ml in the laboratory while in semi-field environments it was 11.91 to 63.63 μg/ml. The LC90 value was found to range between 57.71 to 91.54 μg/ml in the laboratory while in semi-field environments was 52.07 to 173.77 μg/ml. Mortality ranged from 13% to 100% in the laboratory while in semi-field environments it ranged between 43% to 100% within mortality recording time intervals of 12, 24, 48, and 72 hours. The larvicidal activity shown by C. osmophloeum leaf essential oil is a promising alternative to existing larvicides or to be incorporated in integrated larval source management compounds for An. gambiae s.s control. The efficacy observed in this study is attributed to both major and minor compounds of the essential oils.