From the relationship among total fertility rate (TFR), marriage rate (MR) and fertility rate of married women (MFR), this paper studies the cause of declining TFR and discusses the long-term trend of completed fertility rate (CFR) and TFR. First, the decomposition of the declines of TFR shows that it is caused by the declining MR. especially the declining MR of women aged 20-29. Second. CFR from tracing the fertility history of women by cohorts shows that CFR is indeed decreasing. However, even for the cohort born between 1963 and 1967, the total number of birth prior to age 40 is still 1.966, almost 2. Third, data from the 2000 Women's Marriage. Fertility and Employment Survey shows that for the cohort born after 1968, the ideal number of children is decreasing. However, for the cohort born between 1978 and 1982, the preferred number of births is still 1.947. The proportion of women for the same cohort who do not want to have a child is fairly small, only 2.17%. Finally, using data from l980-2005 Manpower Utilization Survey, the educational attainment and the marriage rates by education of 20-34 women show that the educational attainment of women keeps increasing while the marriage rate of college educated women keeps declining. Therefore, it is expected that the marriage rate of women aged 20-29 will still decline in the near future and thus leads to the further decline of TFR. However, whether the trend of declining TFR will eventually be offset by women of the current young cohorts, when they enter marriages in their 30's, needs to be carefully observed.