Analysis in terms of the receiver operating characteristic curve (ROC) has recentely been applied to evaluate medical imaging technique. This paper presents a brief description of ROC and shows how it provides a measure of diagnostic observer performance. Test radiographs of 5 by 5 cm film were shown to 4 observers. Radiographs containing 2 mm 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 curves were analysed. No definite difference of oserver performance between 4 observer and there was superior ROC curve in AO+KH combination than others. The results demonstrate that ROC analysis was useful for evaluating observer performance in detecting task of diagnostic imaging technique.