To investigate the change of cell cycle by genotoxic stress and rebound proliferation in human keratinocytes, the proportions of cell cycle phases were estimated with challenge of UVB irradiation (200 J/m2). With UVB irradiation cell cycle was estimated by Cell Fit program in Flowcytometer, and main change of the cell cycle was S-phase pro-longation. In karyotyping, near diploid number of chromosomes changed to hypoteraploid number. Cell cycle phase was estimated in two groups of cells, near diploid and hypoteraploid. In near dipoid cells, S-phase prolongation was specific phenomenon, while specific G0-G1 phase prolongation was shown in hypoteraploid cells which made transformed foci in culture. The new structural anomalies were del (5q21), 8p+, and t (5 : 8)(q21 : pter). Among them, del (5q21) was found in all transformed hypotetraploid cells. These data suggest that progress of cell cycle could be [G1-S-(G2-G1)-S] by UVB irradiation and deletion of 5q21 has a key role for anchorage independent growth, which is deletion of tumor suppressor gene APC locus. That is one of important mechanisms in keratinocyte transformation by UVB irradiation. With the changes of chromosome number and cell cycle, sizes of nuclei got to bigger by two times and growth rate was delayed.