The paper presents the results of research on an ultrasonic testing technique known as the time-of-flight-diffraction (TOFD) technique. The research-related tests involved a 10 mm thick MMA welded butt joint containing imperfections in the form of linear slag inclusions. The paper contains TOFD images obtained by scanning the face and the root side of the weld. The TOFD examination results were compared with the results of micro and macroscopic metallographic examinations performed at selected points of the welded joint.