This research presents a wind tunnel experiment for investigating three-dimensional flows in the vicinity of a blade in a Horizontal Axis Wind Turbine (HAWT) model. Though the design of the wind turbine blade has been recognized as a modern advance, most of them are based on two-dimensional sectional performance analyses. However, the actual flow around the rotating blade also has a flow effect from a span-wise direction that it is generated from centrifugal and Coriolis forces. A span-wise flow can change the boundary layer on the blade surface. The sectional performance strongly depends on the surface boundary layer. Thus, the actual flow characteristics and correct surface boundary layer in the vicinity of a wind turbine blade is important in designing a wind turbine blade with high performance. In this research, the test wind turbine was a three-bladed type. The test blade comprised four types of airfoils that were smoothly connected and distributed along the blade. The experimental investigation of the flow on the blade surface was performed by simultaneously measuring three-dimensional velocity components by the approach of a three-dimensional Laser Doppler Velocimetry (LDV) method: two LDV probes were used in the synchronized measurement of three-dimensional velocity components. Characteristics of the three-dimensional flow were investigated and visualized by velocity vector field, boundary layer and trajectory path. The results clarified that the three-dimensional flow for the inboard had higher values than the outboard. The two-dimensional relative velocity and the span-wise velocity for the optimum tip speed ratio and low tip speed ratio showed significant differences in the boundary thickness. The shape factor H had satisfactory results and could clearly separate laminar and turbulent regions. The flow trajectory seemed to be affected by the span-wise velocity at chord station y/c > 0.25.