Because mechanical properties of structural materials for high-temperature use, such as boiler tubing, degrade during long-term service, it is essential to detect toughness degradation by means of a nondestructive and simple field test technique. A grain boundary etching technique is developed to detect material degradation, and assess creep strength and notch toughness. An etching test using a picric acid solution with a wetting agent or using 20 percent HNO3 with alcoholic solution was found to have great potential for the nondestructive estimation of grain boundary embrittlement caused by carbide and sigma precipitation in SUS stainless steel. The feasibility of this estimation procedure was determined showing the relationships between Charpy impact energy (CVN) and grooving width (WGS), and creep damage ratio (Φ) and WGS. Superheater tubes of fossil fuel boiler were tested on site to demonstrate the validity of this technique.