The bubbles of water jet cavitation and of multifunction cavitation (MFC) generated by nozzles with diameters of 0.1, 0.2 and 0.8 mm were investigated for the collapse pressure of microjets, the nozzle-specimen distance, the degree of processing, the photocatalyst characteristics for titanium oxide particles, and the multi-bubble sonoluminescence. The dependence of these characteristics on the nozzle diameter was clarified. The nozzle-specimen distance became shorter as the nozzle diameter decreased. The titanium oxide powder was processed with MFC using 0.8, 0.2 and 0.1 mm water jet nozzles. More hydrogen was gen-erated with a nozzle diameter of 0.8 mm than with a 0.1 mm nozzle. A 0.2 mm nozzle generated less hydrogen than a 0.1 mm nozzle because the water jet pressure from a 0.2 mm nozzle was lower. A 0.1 mm nozzle pro-duced the same luminous intensity from cavitation bubbles as a 0.8 mm nozzle. These results mean that the bubble temperature attained by 0.1 and 0.8 mm nozzles is relatively high. The smaller the nozzle diameter becomes, the smaller the bubble diameter and the lower the collapse pressure of the microjet, which leads to a lower degree of processing and reduced influence on photocatalytic properties.