Analysis in terms of the receiver operating characteristic curve (ROC) has recentely been applied to evaluatemedical imaging technique. This paper presents a brief description of ROC and shows how it provides a measure ofdiagnostic observer performance. Test radiographs of 5 by 5cm film, were shown to 4 observers. Radiographscontaining 2mm lucite bead plus noise, or noise only, were obtained with 3 film-screen combinations as AOG+KS,AOG+KH, and AO+KH. Individual ROC curves of 4 observers and 3 film-screen combinations, and pooled ROC cureveswere anlaysed. No definite difference of oserver performance between 4 obserfer, and there was superior ROC curvein AO+KH combination than others. The results demonstraate that ROC analysis was useful for evaluating observerperformance in detecting task of diagnostic imaging technique.