An ultrasonic axial bolting force inspection system is developed for the control of bolts used on turbine outercasings in a thermal power plant. The ultrasonic time of flight through the bolt under stress is analysed using acoustoelastic law. The proportional constant, conventionally called the material constant, is determined by an acoustoelastic constant k and Young's modulus E as (1- kE). Measurements of k for some kinds of steel bolts revealed that k is approximately the same value (-1.1x1O^-11 m²/N) independent of heat treatment. Based on these results, the ultrasonic axial bolting force inspection system is developed. Applying this system to turbine bolts, it is found that the bolts could be controlled more precisely than ever.