The article presents the analysis of guidelines contained in PN-EN ISO 17640 related to ultrasonic tests of welded joints with reference to adjusting the angle of beam insertion into a weld fusion line as well as the results of tests investigating the bevelling angle effect on the amplitude of echo bounced off the bevelled surface simulating the echo bounced off incomplete side fusions. The tests were conducted using a MWB70-4 probe and three specimens bevelled at angles of 20°, 25° and 30°. The article discusses the selection of a beam insertion angle on the detectability of flat discontinuities located on the weld fusion line, inclusive of incomplete side fusions. The article is addressed to NDT personnel, in particular to workers performing ultrasonic testing of welded joints.