Initial susceptibility, normal M-H curve are measured under tensile stress ( sigma less than 40 MPa) for the as-received and heat-treated a-FeBSiC alloy. The heat treatment is done by using infra-red heat pulse. The initial susceptibility decreases with stress and the M-H curve becomes steeper as the stress is applied. Due to heat treatment the magnetic softness increases. From the stress dependence of M-H curve, the magnetostriction constant is calculated and is found to be 16 multiplied by 10** minus **6 and 22. 8 x 10** minus **6 for as-received and heat-treated sample at an excitation field of 20 Oe.