Narrow band imaging (NBI) is a novel technique that enhances the diagnostic capability of endoscopes in charactering tissues by using narrow-bandwidth filters in a red-green-blue sequential illumination system. The blue filter is designed to correspond to the peak absorption spectrum of hemoglobin to emphasize the image of capillary vessels on surface mucosa. By using the red-green-blue images of a hypoglottis, it was confirmed that the blue image represents the network of microblood vessels near the surface mucosa and the green image shows blood vessels at a deeper level. We confirmed that the images of an adenomatous polyp captured by using NBI were clearer than those captured by using a conventional magnifying endoscope and a dye staining. We also confirmed that NBI makes Z lines in Barrett's esophagus more distinct, thereby facilitating observations. We discussed the relationship between the tissue structure of the colon and capillary vessels and the potential for observation, what NBI represents, and potential observation capabilities.