Urea-formaldehyde microcapsules were prepared by in situ interfacial polymerization to lemon oil as the core material using four kinds of emulsifier, gelatin, span 80, polyvinyl alcohol, and sodium dodecyl sulfate. Urea-formaldehyde types were investigated for their effects on thermal properties, mean particle size, and size distributions. Thermal properties were studied using a dynamic DSC and TGA analysis. The morphology and particle distributions of microcapsules were determined using an image analyzer. As experimental results, the diameters and distribution ranges of the microcapsules were decreased with increasing stirring rate and stirring time. In the presence of up to 5 g of emulsifier in 100 ml of deionized water, the mean diameter decreased, and then it increased as the emulsifier increased, resulting from increasing viscosity. The mean size of the microcapsules was related to the viscosity and reunification of core materials. That is, the mean size increased up to 22.8 cP due to increased the viscosity. But, at 30 cP, the mean size of the microcapsules decreased, as a consequence of the interruption of the reunion of core materials by the high viscosity of the core materials.