Using the data of Chinese Longitudinal Healthy Longevity Survey (CLHLS), this study examines the decline process of Activities of Daily Living (ADL) among the Chinese oldest-old born in 1899-1908, 1909-1918 and 1919-1928 by hierarchical linear model (HLM) with mixed effects. This study aims to analyze the age effect, cohort effect, and internal heterogeneity in the process. Results show that there are both age effects and cohort effects in the process of ADL decline among the oldestold. Similar patterns of ADL decline have been found among the oldestold died at different ages. Compared to the chronological age, the remaining survival time before death has stronger predictive power on ADL changes. There are significant cohort differences in ADL changes among the oldest-old. Different socioeconomic statuses, health behaviors, childhood experiences and living environments affect ADL change trajectories among the oldestold, and also contribute to cohort differences.