Hydraulic excavators and cranes are major heavy equipment with numerous one-sided welded joints. Although fillet weld joints are easily implemented and economical, the unwelded portion frequently initiates crack propagation. Therefore, the welded portion is reduced by side welding and by adding a groove. However, attachments with box like structures are difficult to weld at both sides, and grooving cannot completely remove the welded portion because high-quality full penetration welds are difficult to achieve. Despite the need for an accurate evaluation method of crack propagation, the fatigue fracture mechanism and the fatigue damage in practical fillet joints with one-sided welding remains poorly understood. In this study, the fatigue properties and the fracture mechanism of the load carrying type fillet joints with one-sided welding were investigated experimentally to evaluate its fatigue damage with high accuracy based on the experimental results. As the results, fatigue cracks in the test piece initiated from the tip of the unwelded portion and propagated into the welding materials. The three-dimensional observations revealed that fatigue cracks initiate at an early stage of the fatigue development. So, we infer that the fatigue lifetime is chiefly governed by the crack propagation lifetime. Furthermore, from the three-dimensional observation results, cracks were initiated at multiple sites in the test piece. As the number of cycles increased, these cracks propagated and combined.