Abstract: Fe3O4 water-based magnetic fluid of averaged particle diameter 18nm was prepared by chemical co-precipitation and characterized by TEM, VSM and XRD. Test block with a coating artificial crack for different thickness coatings was constructed by machining, welding and grinding processing and so on. We used nano-Fe3O4 magnetic fluids and water-based magnetic HR-1 red magnetic paste that can be bought in the market to the magnetic particle testing of crack defects block, the result shows: with the coating thickness increasing, the magnetic image of both using nano-Fe3O4 magnetic particle and HR-1 magnetic paste was changed as from being dense to being thinning. At the relatively thin coating location, nano-Fe3O4 magnetic image resolution was a bit better than the conventional HR-1 red magnetic paste, but at the thicker location, nano-Fe3O4 magnetic image was slightly superior to the conventional HR-1 red magnetic paste while applying an extra magnetic field, but when the magnetic field was moved away, nano-Fe3O4 magnetic image became loose, discontinuous, so the display effect was no better than conventional HR-1 red magnetic paste.