Ice formation in a horizontal circular has been studied numerically. From the numerical analysis results, it was found that there were three types of freezing pat terns and that the freezing phenomenon was affect ed largely by densit y inversion and cooling rate. The type of freezing patt ern largely depends on the secondary ow, which is generat ed by density inversion. W hen supercooling energy is released before the development of the secondary ow, an annular ice layer grows. If the energy is released when the secondary ow is considerably developed and the supercooled region is removed to the upper half part of the cylinder, an asymmet ric ice layer grows. If the energy is released after perfect development of the secondary ow, instantaneous dendritic ice format ion over the full region occurs. Furthermore, the secondary ow was found to have an effect on heat transfer charact erist ics. The heat transfer rate becomes small at the instant when the secondary ow is generated, but it becomes large with the development of the secondary ow. It is concluded that for the facilitation of heat transfer it is desirable to keep wat er, in its liquid phase until the secondary ow is perfectly developed. This study gives an instruction on the performance improvement of a capsule-type ice storage tank.