A one-step direct method of making a high molecular weight functionalized polyolefin aqueous emulsion is provided. In this one-step direct method, a functionalized polyolefin having a molecular weight of at least 10,000, a fatty acid, a base, a surfactant, and water are heated in a pressure reaction vessel to a temperature above the emulsification temperature of the polyolefin with agitation for a period of time sufficient to form an aqueous emulsion. This high molecular weight functionalized polyolefin aqueous emulsion can be added to a sizing composition and applied directly onto glass fibers in the glass fiber manufacturing process.