A new detector system using multi-anode photomultiplier tubes has been developed for measurement of an annihilation point of ortho-positronium (oPs). Position resolutions of the detector with YAP:Ce and CsI(Tl) are determined to be 4.0 and 5.7 mm, respectively. A recently developed YAP:Ce scintillator might be a good candidate for our experimental purpose with respect to high light-output, short decay time and excellent stopping power. Extensive Monte Carlo simulation studies have been performed to understand utility of this system for measuring the annihilation points of oPs. (C) 2002 Published by Elsevier Science B.V.