In spite of the developments of various autostereoscopic three-dimensional (3D) technologies, the inferior resolution of the realized 3D image is a severe problem that should be resolved. For that purpose, a time-sequential 3D display is developed to provide 3D images with higher resolution and attracts much attention. Among them, a method using a directional backlight unit (DBLU) is an effective way to be adopted in liquid crystal display (LCD) with higher frame rate such as 120Hz. However, in the conventional time-sequential system, the insufficient frame rate results a flicker problem which means a recognizable fluctuation of image brightness. A dot dithering method can be a good solution for reducing that problem but it was impossible to observe the 3D image in side lobes because the image data and the directivity of light rays from the DBLU do not match in side lobes. In this paper, we propose a new vertical line dithering method to expand the area for 3D image observation by utilizing the side lobes. Since the side lobes locate in the left and right position of the center lobe, it is needed to arrange the image data in LCD panel and directivity of the light rays from the DBLU to have continuity in horizontal direction. Although the observed 3D images in side lobes are flipped ones, the utilization of the side lobes can increase the number of observers in horizontal direction.