This article which is the first part of the demographic and eco-historical study on southeast China during the Ming-Qing period analyses three genealogies of the higher-order lineages inhabiting Shanyin (山陰), Zhuji (諸曁) and Jiangshan (江山) counties in order to estimate the degree to which the seasonal cycle influenced the mortality. Depending on these genealogies which give us the information on the date when the member of the lineages died, we can calculate the monthly fluctuations in death for every term of fifty years from 1451 to 1850 (tables 1, 2, 3). The range of indicators is very wide in the first three terms (1451-1600), but on the other hand we can find little fluctuations after 1750. This change in the pattern of mortality indicates that a demographic revolution occurred in seventeenth century. Each graph (figures 2, 3, 4) which illustrates the monthly fluctuations in death for each lineage before 1650 has two peaks and two troughs; the graph for the Shanyin Zhu (朱) lineage has troughs in the 5th-6th and the 11th-1st months of lunar calendar, that of the Zhuji Zhong (鐘) has troughs in the 6th and the 12th-1st months, and that of the Jiangshan Mao (毛) has troughs in the 5th and the 9th months. The pattern of the graph reflects the agricultural cycle of the area where each lineage lived. Shanyin where the Zhu lineage was located was in the corner of the Ningpo-Shaoxing Plain where peasants had two harvests, early and late rice crops, in the 5th-6th and 11th months of the lunar calendar. Zhuji county consisted of hills and alluvial flat which were suited to the rotation of rice and winter wheat reaped in the 6th month. Jiangshan county was a mountainous district where late rice and winter wheat were planted. The death rate dropped just after harvest times. There is little point in focusing research efforts on why the pattern of mortality changed in seventeenth century, since cause and effect were interwined in a very complex relationship within the social structure which it is almost impossible to disentangle, What we sould examine is relationship between the demographic revolution and various phenomena, for example, the structural change of lineage and administration, the introduction of New World crops, the improvement of the monetary economy and the damage to the ecological environment.