A critical plane model for the fatigue limit of metals with small defects is developed based on the square root of area parameter. The model is designed to reflect the Mode I-dominated physical damage mechanism of small defects. The concept of directionally dependent fatigue strength is introduced to extend the critical plane definition to defects whose projected area varies with the plane. The Walker relation with a critical plane interpretation is proposed to account for the mean stress effect. The model is evaluated using available experimental data of steels containing artificial surface defects and ductile cast iron having inherent graphite nodules. The experiments include different proportional and nonproportional axial-torsional loading conditions and defect types (cylindrical, hemispherical, and tilted hemiellipsoidal holes). The model is found to give good estimates of both fatigue limits and crack directions. A discussion on the physical interpretation of critical plane models in the context of the small defect fatigue problem is presented.