Polyurethane (PU) microcapsules containing hexamethylene diisocyanate (HDI) as core materials are facilely manufactured via interfacial polymerization reaction of commercial methylene diphenyl diisocyanate (MDI) prepolymer and 1,4-butanediol in an oil-in-water emulsion. The resultant capsules have diameters of 5–350 μm and shell thickness of 1–15 μm, both linearly related to the agitation rate in the double logarithm coordinates. The typical core fraction of microcapsules and the yield of synthesis are around 60 wt% and 70 wt%, respectively, while varying with reaction conditions. The effects of parameters including reaction duration and temperature, surfactant concentration, agitation rate, and environmental factors on the formation of microcapsules were systematically investigated and optimized. Quality assessments of each batch of microcapsules were performed using thermogravimetric analysis and scanning electron microscopy. Anticorrosion coatings mixed with synthesized microcapsules were prepared on a steel substrate. Preliminary results indicated significant corrosion retardancy happened in the self-healing coatings under an accelerated corrosion process, showing the great potential of this facile microencapsulation technique in development of catalyst-free, one-part self-healing coatings for corrosion control.