In this study, g-C3N4 was hydrothermally synthesized at 200 °C for 4–24 h and then calcined to produce g-C3N4 microtubes. As the hydrothermal synthesis time increased, the morphology changed from particles to microtubes. In addition, the crystal structures of synthesized g-C3N4 microtubes were evaluated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results showed that g-C3N4 synthesized by 4 and 8 h hydrothermal processes were particulate, whereas g-C3N4 synthesized by 12 and 16 h hydrothermal processes were tubular. Tubular g-C3N4 was found to have numerous holes on its surface. When the g-C3N4 powders were placed in water, only the 16 h synthesized powder could float because the density of g-C3N4 microtubes hydrothermally synthesized for 16 h was less than that of water. The photocatalytic activity of the g-C3N4 microtubes was evaluated using methylene blue degradation experiments. The results showed that g-C3N4 microtubes with 12 h hydrothermal synthesis showed the highest degradation rate. This result may be because the g-C3N4 microtubes have a large surface area because of their tube structure, and they also sink into the liquid, allowing them to come in more contact with the methylene blue solution. The g-C3N4 microtubes with a 16 h hydrothermal synthesis showed the second-highest degradation efficiency while floating on the surface of the methylene blue solution. These g-C3N4 microtubes are attractive because they can receive more light energy and can be collected easily after the experiment because of their ability to float on the liquid surface.