Fatigue crack propagation experiments were carried out on a mild steel, its heat-affected-zone (HAZ) and weld metal. A full thickness side notch of radius 0. 5 mm, width 1 mm and depth 14 mm was machined on both sides of a specimen 10 mm thick and 70 mm wide. For HAZ and weld metal specimens notch center was made to locate at the centerline of the material, that was made by manual welding of right-angled triangular type edge preparation. Zero-to-tension repeated axial stress, which was less than 10. 0 kg/mm², was applied on a specimen, and fatigue crack growth rate was measured on both surfaces of a specimen with the aid of a magnifying glass. Macroscopic crack growth rates showed a dependency on stress intensity range, Δ K , following the relation of C (Δ K ) ^m. The mean value of the power m could be determined to 1. 7 with little deviation, while constant C was observed as a function of stress range. No significant difference in fatigue crack growth rate behaviour was distinguished among base metal, HAZ and weld metal. Comparison of the present results on the dependency of log C on m with the relations proposed by Kitagawa, Koshiga and Gurney-Maddox showed a crossing tendency to the previous relations. Fractographic investigation on fatigue fracture surfaces revealed the following : (1) Distributions of striation spacings are followed to normal distribution, (2) No significant difference in topography is observed among base metal, HAZ and weld metal, (3) Featureless topography is observed at a point approximately 0. 2 mm inside from a specimen surface, (4) Striation spacing defined at a point approximately 0. 4 mm inside from the surface is quite same as that in the midthickness, and (5) The value of 1. 3 is obtained as an approximate value of the exponent in the relation s = C _s (Δ K ) ^ms.