This master thesis is focused on non-destructive methods of testing machine part KP.17-DI.94. The procedures of non-destructive methods with whom the part can be tested are described in theory. Magnetic particle inspection method is described more thoroughly and has been chosen for practical part of this thesis. Characteristic discontinuities in welded joints are listed and described according to the norm HRN EN ISO 6250-1 so they could be more easily detected when testing. In practical part of this master thesis, the machine part KP.17-DI.94. was tested with magnetic particle inspection method. Machine part was a steel plate made of 15 mm thick S355 material and welded with Metal Active Gas procedure. The welded joint is the subject of this test. Two types of magnetic particles were used and the test was conducted according to the norm HRN EN ISO 17638. With the analysis of results irregularities characteristic for MAG welding procedure were found. According to the norm HRN EN ISO 23278 accepted levels of the irregularities were determined. The results of magnetic particle inspection method were compared with the results of radiography testing. The conclusion was that the testing was successful and that the magnetic particle inspection method is the best for detection of surface irregularities.