Kilohertz frequency alternating current (KHFAC) stimulation can induce fast-acting, reversible and repeatable nerve conduction block, and is a candidate therapeutic method for diseases caused by undesired neural activities, such as urinary retention. In this paper, we first show that ultra-high frequency (UHF) current pulses can also lead to successful nerve conduction block, based on simulation results using the McIntyre-Richardson-Grill (MRG) model. This model describes a myelinated axon of mammalian animals. Second, we present a prototype of a power efficient neural stimulator using UHF current pulses with active charge balancing (CB). The stimulator is built using off-the-shelf components and can be battery-powered. It uses a DC-DC boost converter without a big filtering capacitor, for generating UHF current pulses. The power efficiency of the complete system is up to 98% when testing with an equivalent circuit model of electrode tissue interface (ETI). Safety measurement results show that the electrode offset voltage can be as high as 1.3 V without charge balancing, in in vitro experiments with titanium electrodes in a phosphate buffered saline (PBS) solution. However, this electrode offset voltage can be successfully lowered to less than 42.5 mV, by means of negative-feedback duty cycle control of the H-bridge clock. The active CB is adopted for KHFAC stimulation for the first time.