In this paper, uniform spherical poly(urea-formaldehyde) (PUF) microcapsules containing paraffins, which can be used as phase change materials for energy storage, were prepared by in situ polymerization method under high-speed agitation (a parts per thousand yen10,000 rpm) without emulsifier. The influence of high-speed agitation on particle size of as-prepared microcapsules and the tightness of microcapsules were also investigated. The results show that, all the microcapsules have < 10 mu m mean particles-size and narrow-size distribution, and the mean particle size decreases with the increase of agitation rate. Furthermore, when the agitation rate is > 16,000 rpm, the effectiveness of reducing particle size by high-speed stirring is not as remarkable as that of lower speed agitation. In order to gain good tightness of PUF microcapsules under the high-speed agitation conditions, the final pH value of reaction solution should be lower down compared with that of conventional agitation. In our investigation, when the agitation rate was 10,000 rpm, microcapsules fabricated at pH value < 2.0 were sealed and own good tightness, however, those fabricated at pH value > 2.2 were not sealed.