A serial printer using a synchronous belt in the print-head carriage system generates a vertical striped pattern in the printing image. This is known as jitter and can be considered to be caused by vibration of the print-head carriage system. In this paper, as the first step in the study of jitter, a method of numerically evaluating jitter is proposed and a vibration experiment of the carriage system carried out. An image scanner is used to digitize the printing image and a relative brightness fluctuation is introduced to express jitter. The frequencies and the magnitudes of jitter are clarified by spectrum analysis. The results of the vibration experiment shows good coincidence between the vibration velocity of the carriage system and jitter. It is found that all vibrations in the length direction of the carriage system appear on the printing image as jitter.