The deterioration conditions of nuclear power plants and chemical plants have raised deep concerns about the safety of high-risk structures. The safety of a structure can be secured by evaluations of breakages and lifespan in the early stage. Therefore, the material condition of a structure needs to be estimated. A nonlinear ultrasonic evaluation method is reportedly more sensitive on microscopic changes of a material than a linear evaluation method. Therefore, the nonlinear ultrasonic technique based on ultrasonic nonlinearity is considered a promising method of evaluation for breakages and lifespan. This study measured the changes in nonlinear parameters by using a nonlinear ultrasonic method, observed the microstructure of a heated material, and analyzed the relation between the size of the microstructure and the changes in the nonlinear parameters.