The "arc-in-liquid" method is a simple and inexpensive technique for the synthesis of carbon nanotubes and related nano-materials. In this paper, we report on the synthesis of carbon nanotubes by means of the arc-in-water method under microgravity and normal gravity conditions. The heat of convection and two-phase flow caused by the arc plasma are suppressed under microgravity, so the heat and fluid flow are stabilized under such conditions and a single huge bubble is generated around the electrodes. From the images captured during the experiment of the arc-in-liquid method, it can be observed that the bubble contained a layer of water vapor at the gas-liquid interface under microgravity conditions, and this layer blocked the carbon vapor reaching the liquid phase. Owing to this unique phenomenon, it was determined that the synthesis of carbon nanotubes by the arc-in-water method is strongly affected by gravity.