This paper presents a design methodology for the digital controllers of a three-phase UPS (Uninterruptible Power Supply). Upon a state-feedback control topology, a systematic method for pole placement and determination of controller gains was developed. In order to attest the efficiency of the adopted control scheme, experimental tests were carried out on a 4kVA three-phase on-line UPS, resulting in unity power factor operation of the controlled rectifier and low harmonic distortion of inverter output voltage, even under heavy non-linear loads.