When dye penetrant testing method is used to detect ZG04Cr13Ni4Mo stainless steel castings ma-chined surface quality，we are able to observe round spots． In order to study whether such displayed dots indicatethe presence of defect on machined surface，and which defect types they correspond to，penetrant testing methodshave been adopted to detect machined surface defects of ZG04Cr13Ni4Mo stainless steel samples，and scanning e-lectron microscope and metallurgical microscope are used to observe and analyze displayed dots． Test results showthat displayed dots were observed on the sample surface． Dot size is in the range of 0. 07 ～ 1. 2 mm． After the sam-ples are cleaned，scanning electron microscope was used to observe and analyze the imaging areas on sample sur-face． Defects are found on sample surface，which belong to inclusion defects． Defect size is in the range of 0. 07 ～1. 2 mm． These inclusions are rich in O，Si，Al，and Ca four elements．