Abstract: As it is widely known that the story number affects to the building period during earthquake shaking. However, how far the affection is and what other influencing structural parameters are needs to be intensively studied. This study explores the characteristic of the horizontal motion of three mid-rise buildings using modal analysis method. The method is used to calculate the modal parameters of the buildings to evaluate the motion characteristics of the buildings. It has been found that when the story number of a building increases: the building will become more flexible as indicated by the lengthening the modal periods, the building will vibrate longer as indicated by the decreasing the modal damping ratios and the domination of lower modes to the building vibration decreases as indicated by increasing the summation of the absolute values of the modal participation factors for high modes.