Abstract: A rapid and reliable non-destructive method is proposed for the determination of stress in a steel material by determining the reversible magnetic permeability .mu. under a magnetic bias field H.sub.0 in the range of approach to the saturation magnetization, calculating the factor M in the equation .mu.=1+M/H.sub.0.sup.3 by using the value of .mu., and calculating .sigma..sub.1, .sigma..sub.2 and .theta. by solving the equation ##EQU1## in which .sigma..sub.1 is a principal stress, .sigma..sub.2 is the principal stress perpendicular to .sigma..sub.1, .theta. is the angle between .sigma..sub.1 and the direction of measurement of .mu., a is the value of .sqroot.M in a stress-free state and b is the stress sensitivity, a and b being determined separately.