A cylindrical sonochemical reactor made of an acrylic pipe was investigated under sonication at the frequency of 490 kHz. The sonochemical efficiency depended on the distance between the measuring position and the transducer. Sonochemical luminescence and oxidation of V ions in aqueous solutions were observed at the upper part of the reactor. The liquid temperature in the reactor increased with the irradiation time, and the estimated ultrasonic power was independent of the measuring position. From these results, the sonochemical efficiency in the cylindrical reactor was estimated.