A series of microcapsulescontaining n-octadecane with a urea-melamine-formaldehyde copolymer shell were synthesized by insitu polymerization. The surface morphology, diameter, melting and crystallization properties, and thermal stability of the microcapsules were investigated by using FTIR, SEM, DSC, TGA and DTA. The diameters of the microcapsules are in the range of 0.2-5.6µm. The noctadecane contents in the micro-capsules are in the range of 65-78wt%. The mole ratio of urea-melamine has been found to have no effect on the melting temperature of the microcapsules. Two crystallization peaks on the DSC cooling curve have been observed. The thermal damage mechanisms are the lique-fied n-octadecane leaking from the microcapsule and breakage of the shell due to the mismatch of thermal expansion of the core and shell materials at high temperatures. The thermal stability of materials can be enhanced up to 10℃ by the copo-lymerization of urea, melamine and formaldehyde in a mole ratio 0.2:0.8:3. The thermal stability of 160℃ heat-treated microcapsules containing 8.8% cyclohexane can be further enhanced up to approxi-mately 37℃.