Abstract: This paper presents an analysis of open cavity of C-band MILO. The eigenfrequency of the open cavity and Q factor of loaded-cavity are calculated by observing the response of the broad-band driver. The numerical method based on the finite difference method in time domain is applied to calculate the radial electric field of the open cavity within one period. The distribution of the field is divided into four regions, the cut-off region, the standing wave region, the transition region, and the traveling-wave region. The inherent Q factor and the loaded Q factor are also calculated.