The effect of both compressive and tensile stresses of up to 500lbf/in2 at various temperatures in the range 20–400°C on the power loss, coercive force and remanence of two grades of grain-oriented 3% silicon iron has been measured at 50c/s. Increasing tension at first reduces the power loss and the coercive force, but at tensions above a certain value both are increased. The degree of improvement in magnetic properties. obtained under tension depends on the grade of material. Increasing a compressive stress causes both power loss and coercive force to rise monotonically; a compressive stress of 400lbf/in2 gives an increase in power loss of about 30% for the better grade of material and about 20% for the other. The effect of increasing the temperature is to cause the loss to fall by a few percent for every 100 deg C change; the amount varies little with the applied stress. Both the variation of the power loss and the variation of the static hysteresis loss with stress follow a similar pattern.These results are of great importance to the power-transformer engineer. Compressive stresses in the plane of the lamination and in the direction of rolling are generated by clamping nonflat coreplates during construction of a core and by temperature gradients across the core limbs.