Objective　To provide the accurate radiological anatomical evidence for clinical diagnosis of lesions of the sacroiliac joints by studying the radiographic anatomic features of the SIJ on the anteroposterior and oblique plain radiography.　Methods　Iliac and sacral auricular surface of each male and female dry adult plevics evenly were coated with barium powder, folded and fastened with the tape and and underwent anteroposterior and oblique plain radiography of the pelvic; then the barium powder was washed away, the edge of the sacral and iliac auricular surfaces were surrounded with thin lead wires, foldedand fastened with tape, and underwent the anterposterior and oblique radiography of the pelvis. the projective changes and morphological characteristics of SIJ and its auricular surface in the anteroposterior and oblique radiography were observed.   Result    The anterior and posterior spaces of sacroiliac spaces and auricular surfaces were clearly displayed in the anteroposterior radiography. the contralateral SIJ could be more clearly displayed in the oblique radiography. The sacroiliac synovial joints were located in the inferior 2/3 of the sacroiliac space.    Conclusions    The radiological anatomic and gender characteristics of SIJ are presented in the anterposterior and oblique radiography of the pelvis. The observation should focus on the anterior and posterior spaces and the scope of auricular surface of SIJ joint. This study indicates that the anteroposterior radiography is enough for meeting the clinical requirement.