A conventional battery-powered AC uninterruptible power supply (AC-UPS) that steps up low DC voltage to high DC voltage and cascades with the high frequency inverter is complicated in control and of low efficiency due to two-stage power conversion. This paper presents a three-phase high step-up converter with a single-stage DC/DC power conversion suitable for micro DC-UPS applications. In this paper, the circuit operating theory and steady-state analysis of the proposed topology are first addressed then a 100 V/110 W circuit prototype is designed, built and applied in 24 V-battery-powered micro DC-UPS for a 19 V/85 W laptop PC appliance. The feasibility and effectiveness of the proposed converter topology are confirmed with experimental results. The maximum conversion efficiency of the proposed converter is about 93.5%, and the overall efficiency of the designed system is high due to the losses in the power conversion process when reduced. Compared with the conventional AC-UPS, the proposed micro DC-UPS achieves potentially about 3% maximum energy savings.