Abstract: NDT(nondestructive testing)techniques were employed to detect cavity defects in timbers. Therefore it is necessary to make comparisons on those NDT techniques in order to find out their special feature and suitable application, so that the interior defects information of timbers were judged correctly. In order to compare the precision of electric resistance tomography(ERT)and stress wave technology(SWT), under the condition of different area and outline of simulated cavity defects in timber discs of Cunninghamia lanceolata, the relationship between imaging graph defects detected by two methods and real defects was studied. Research results showed that: ①Both ERT and SWT could display graph of defects, the precision of imaging graph relates to real defect area and area of the tested wood cross section, and outline shape of the defects; ②When the ratio between real defect area and area of the tested wood cross section raisedy from 4.90% to 44.05%, the relative error of ERT graph defect area and real defect area dropped from 34.03% to 11.69%, and the relative error of SWT graph defect area and real defect area dropped from 46.41% to 14.88%. ③ Outline shape of defects exerted certain effect on detection of defects of both imaging systems. The defects of semi-round shape were easy to shown by ERT. On the contrary, the defects of long and narrow fan shape were easy to be shown by SWT. This study indicated that ERT was more sensitive than SWT, and ERT had a higher precision in image detection than SWT. On the other hand, SWT performed better in identification capabilities of defect outlines.