A precise method for estimating the axial stress is proposed for the short and highly stressed bolts used for automobile. These bolts show non-linear extension, because of local plastic deformation as well as non-uniform stress distribution in bolt cross section. The proposed method consists of(a)precise measurement of the time-of-flight of longitudinal and transverse waves using a digital pulse-echo technique, (b)to establish the calibration curves which take into account of nonlinear extension due to non-uniform stress distribution based on simultaneous measurement of the axial elongation, time-of-flight of longitudinal and transverse waves and(c)evaluation of the axial stress with the change in the time-of-flight and the assumed length of equivalent stressed length. The axial stress is estimated within 1% error for one type bolts and within 2% error for another type bolts.