A herbicidal composition comprises microcapsules having a polymer shell and containing therein clomazone and at least one alga. Preferably the shell of the microcapsules comprises polyurea. Also claimed is a method of preparing a herbicide composition, the method comprises the steps of: providing a water immiscible phase comprising clomazone, an alga, an isocyanate and optionally an acetylene carbamide derivative (ACD) cross-linker； providing an aqueous phase comprising one or more surfactants； combining the water immiscible phase and the aqueous phase to form a dispersion of the water immiscible phase in the aqueous phase； thereby forming microcapsules of polyurea containing droplets of the water immiscible phase； and curing the microcapsules.
