Objectives: This study was to investigate the reciprocal relationship between frailty and physical activity among older adults by age group (middle-old: 70-79 years; oldest-old: 80-84 years) within 2 years using cross-lagged panel analysis. Methods: The study data were derived from the Korean Frailty and Aging Cohort Study, and a total of 1092 participants were included. Results: Frailty and high physical activity had significant reciprocal relationships in the middle-old group, which indicates that frailty was associated with less high physical activity, and high physical activity predicts less frailty after 2 years. In the oldest-old group, there was no statistically significant reciprocal relationship between frailty and any level of physical activity reference to low physical activity and vice versa after 2 years. Discussion: Further studies on the relationship between frailty and physical activity of the oldest-old population and specific physical activity guidelines for older adults are needed.