Population momentum and population inertia are connected concepts. Both are used to describe the phenomenon that the population will continue growing or decreasing before reaching a stationary population when the fertility reaches the replacement level. However, there are also differences. The population momentum is virtual and can be estimated for any year, while population inertia is a specific demographic phenomenon that only occurs after the population's fertility reaches the replacement level. The Lag effect of fertility on generation replacement is the fundamental cause of population inertia and determines that the standard period of inertial period is equal to the difference between the population's life expectancy at birth and the mean age of childbearing. The length of an actual inertial period of the population is affected by the fertility level during the inertial period and the size of migration. The amplitude of population growth or decrease during the inertial period is affected by the fertility rates in the decades before entering the inertial period. Empirical analysis shows that China, Japan, South Korea and Germany have already undergone the complete processes of population inertial growth which have lasted for 30-50 years. The UK, France, Thailand and Brazil are in the process of population inertial growth. So far, no country in the world has entered the process of negative inertial growth of population.

China’s population is at a turning point.By calculating intrinsic rate of natural increase over 1950-2006,this paper shows that the intrinsic rate turned negative in as early as 1990 despite the positive natural increase rate of China’s population.China has been gaining negative population growth momentum as a result of low fertility over the past decade.Population simulations under different scenarios demonstrate major impact of the negative momentum on China’s future size and age structure of population.Policy implications are discussed.