As a novel vircator device, the coaxial vircator can attain much higher efficiencythan the common virtual cathode oscillator. In this paper, coaxial vircators in three specificconfigurations are studied to describe the efficiency and frequency characteristics and their depen-dence on geometric parameters. The PIC (Particle in Cell) simulation results show that a powerefficiency over 11% can be obtained from the coaxial vircator with a finite inner anode conduc-tor, and a narrow-band output can be achieved. A direct-coupled coaxial vircator has a slightlylower efficiency of about 8%, but in this configuration the efficiency is not strongly dependenton the geometry parameters. The introduction of a reflecting cavity can effectively improve theefficiency under certain parameters. Meanwhile, the research results also indicate that impropercavity parameters will countervail some inherent advantages of the configuration with a finiteinner conductor, leading to lower efficiency than that of the direct-coupled one. Meanwhile, anexperiment was also carried out to testify the simulation results, and a power efficiency over 3%was obtained from the direct-coupled coaxial vircator.