In the past, the main concern of population-related issues in Taiwan has been to control the natural growth rate and to mitigate its potential negative impacts. In the experience of other countries, a rapidly declining birth rate is associated with population aging. It took spent only about 70 years to complete its demographic transition. By the end of 1993, population aged 65 and over had reached a level of about 7 percent. Thus, it is expected that population aging shall become a major national concern in the future. Based on the existing demographic data, the paper calculates the growth rate of the elderly and an Aging Index, aiming at analyzing the spatial differential of population aging in Taipei County, the most populous in Taiwan. The research finds that 1) population aging is related ,to the level of urbanization: rural areas tend to be associated with a higher level of aged ,population than their urban counterparts; and 2) the level of population aging is negatively ,related to the level of regional economic development: because of better job opportunities, ,the new towns along Route 3 attract rural youths, mitigates the problem of population aging ,in those areas, while population aging is a growing concern for rural areas.