For the plunging water jet system using inclined short nozzles, the flow characteristics such as the bubble penetration depth and the gas entrainment rate, which changed depending on the jet velocity, the nozzle diameter, the jet length, and the jet angle were first evaluated in an air-water system. A comparable investigation between our results and those of existing studies used the long nozzles on those characteristics revealed that both the bubble penetration depth and the gas entrainment rate differed depending on the nozzle length; that is, the nozzle-length-to-diameter ratio L(N)/D(N) and that of these characteristics the gas entrainment rate affected considerably by its magnitude and tended to be high when the nozzle of a large L(N)/D(N) ratio was used. It was also confirmed from the oxygen transfer experiments that the transfer efficiency at low jet velocities in the present water jet system was not inferior to the ones of other types of existing aeration systems; that is, the utilization of this jet aeration system to a high rate reactor for wastewater treatment or fermentation was sufficiently possible. The applicability of the plunging jet aeration method to microbial processes was then examined. As a typical example of microbial processes to be tested, the continuous treatment of an organic wastewater using activated sludge microorganisms was carried out, and the performance and related problem when this type of aeration system was applied to such a microbial process were investigated. Experimental results showed that, when viewed from the removal ability of dissolved organic matters, the plunging jet aeration system was capable of treating a wastewater of considerable high loading without the rate of oxygen transfer becoming the biooxydation-rate-limiting factor. Special attention was necessary for the choice of the liquid pump to be employed, however, due to the increased amount of fine suspended solids in the treated water caused by the shearing action between sludge flocks and pump blades.