The average value of the crack tip stress intensity factor produced by a row of impurity atoms is calculated in this paper. The row of impurity atoms lies parallel to the crack front and is situated at an arbitrary position about a crack. The impurity atom produced mode I shielding/antishielding stress intensity factor LiI for a row of impurity atoms very close to the crack tip is given by the expression: 295-1 where 2a is the crack length, a* is either the atomic radius of the host atoms (for substitutional impurities) or the radius of the vacant interstitial site (for interstitial impurities), 295-2 and 295-3 where G is the shear modulus, c is the fraction of impurity atoms in the impurity atom row and e is the misfit parameter of the impurity atoms, r2 is the distance from the crack tip, and 295-4 is the angle the radius r2 makes with the crack plane.