The Japanese Black cattle population in Hyogo prefecture has been maintained as a closed herd, and this supplies young sires and heifers for the whole population of the breed. The population structure in Hyogo prefecture was investigated by pedigree analyses. Population genetic parameters. Such as the index of diversity of sires, the effective sire number, F-statistics (F-it, F-st and F-is), the index of subdivision and the effective population size, were estimated using random samples of 200 Japanese Black registered heifers produced in Hyogo prefecture in 1960, 1970, 1980 or 1988. The diversity of sires greatly decreased from 1960 to 1970 and from 1980 to 1988. the average inbreeding coefficient (F-it) and the average inbreeding coefficient expected by random mating (F-st) increased year by year, and F-st became higher than F-it from 1980. Thus the inbreeding coefficient due to consanguine mating (F-is) and the index of subdivision gradually decreased. The effective population size decreased from 262 in 1960 to 8 in 1988. It is concluded that the genetic homogeneity and the reduction of the population size occurred due to the spread of artificial insemination in the Japanese Black cattle population in Hyogo prefecture, and this trend was more strengthened recently