In this study, the seasonal variation of Earth's rotational velocity is studied from a dynamical point of view. Many studies have been conducted in recent years by methods of correlation analysis, dynamical calculation, and numerical simulation. In this paper, we use an analytical method that has not been attempted in this study area to establish a model in which the angular momentum conservation, short-wave and long-wave radiation, the radiation transmitting process, and the anomaly of atmospheric circulation are considered. On the basis of the angular momentum conservation law of the Earth-atmosphere system, we obtain an analytical solution for variation of Earth's angular velocity. Our model shows that the semiannual variation is mainly caused by solar short-wave radiation and that the annual change is generated by short-wave radiation from the Sun and long-wave radiation from the atmospheric underlying surface. Our model also shows that long-wave radiation from the atmosphere affects the nonuniformity of Earth's rotation by reducing amplitude and changing phase. The results suggest that the action of ocean for seasonal variation is unimportant, that the nonlinear action of zonal averaged field can be neglected, and that only a secondary part of atmospheric circulation anomaly is decisive for the seasonal variation. Our analytical solution depends on general astronomical parameters and therefore can be applied to other planets. ¿ American Geophysical Union 1995