In ozone production using plasma generated with pulsed power, the concentration of ozone is not high although ozone is produced with high efficiency. Effect of shortening electrode separation of a coaxial reactor was investigated in order to generate ozone in high concentration in using pulsed power. Ozone concentration and yield increased with decreasing diameter of the reactor because spatial density of plasma would become higher. In these reactors, the ozone yield remained in nearly constant value while the ozone concentration increased in proportion to pulse repetition rate. In reactor of 17 mm, saturation tendency and reduction of the concentration were however seen. In this reactor, the ozone concentration decreases with time after reaching a peak value. Spark discharges with short electrode separation caused temperature elevation of electrodes and gases, so that thermal decomposition of ozone molecules occurred. When oxygen gas containing ozone instead of pure oxygen gas flowed into the reactor, rise of the ozone concentration competed with ozone concentration produced with pure ozone gas. The rise showed potential of plasma for ozone production. Ozone with higher concentration could be obtained by the potential of plasma on an appropriate condition.