The nondestructive testings for structural fine ceramic materials have been evaluated by film method. Thickness effects on the detectability of neutron radiography have been studied theoretically and experimentally. Both methods with and without contrast enhancer are studied by using Kyoto University Reactor facility. The experimental detectabilities obtained have been in good agreement with the theoretical analyses. By using contrast enhancer, the small defects on the surface of silicon nitride ceramics being near 15 microns in width and depth have been clearly detected. For the case of non-enhancer, 0.6% of the detectability has been obtained over the thickness of 30 to 40 mm for the silicon nitride ceramics