The size, size distribution, and stability of hydrogen nanobubbles generated in the catholyte of a potassium carbonate solution, i.e., alkaline electrolyzed water, were determined using a dynamic light scattering method. Results show the following: 1) Hydrogen nanobubbles existed stably for 24 h in the closed system. The average size of the nanobubbles 5 h after their generation was 160 nm. It was 184 nm after 24 h. Some of them combined to form 5–10 µm diameter bubbles. 2) In an open system, hydrogen nanobubbles were present even after 24 h; the Ostwald ripening phenomenon was observed. That is, although some nanobubbles dissolved and vanished into the solution by ripening, others grew to over a micrometer. Therefore, their distribution range widened during ripening.