Microcapsules and nanocapsules containing n-octadecane with melamine-formaldehyde shell were fabricated by in situ polymerization. The effects of stirring rate, contents of emulsifier and contents of cyclohexane on diameters, morphology, phase change properties and thermal stabilities of the capsules were studied by using FT-IR, SEM, DSC and TG. The diameters of the microcapsules decreased with the increase of stirring rate, emulsifier content and cyclohexane content. The relationship between stirring rate and number-average diameters or weight-average diameters of the microcapsules is an exponent. The relationship between emulsifier content and number-average diameters or weight-average diameters of the microcapsules is an exponent, too. The diameters had less effects on the melting behaviour of microcapsules, however, they had significant effects on the crystallization behaviour. As the diameters of the microcapsules decreased, two crystallization peaks were observed from the DSC cooling curves. The ratio of the integrated area of the peak in the lower temperature increased with the decrease in diameters of microcapsules. The thermal stability of micro/nanocapsule rises with the increase of stirring rates and TA content. The cyclohexane content has no significantly effect on thermal stability of the nanocapsule.