Abstract: The hydrogen bubble flow visualization technique was used to study the effects of small semisphere on the boundary layer transition of flow over a smooth circular plate. The experimental results indicated that the periodic hairpin vortex will shed from the semi-sphere when Rer( Reynolds number based on the radius of the semi-sphere r) was greater than 302. Due to the effect of self-induced velocity, the hairpin vortex lifted with an inclined angle. The head of the hairpin vortex moved into the higher velocity region first, so the head possessed a higher velocity than the leg part of the hairpin vortex. The hairpin vortex was strentched at the streamwise direction, which increased the streamwise vortex strength and led to much higher upward velocity of the vortex head. The continuously created strentched and self-induced processes led to the three dimensional disturbance growing quickly, so the velocity profile of the boundary layer was suffered from the temporal local strain. The strong unstable shear layer was created, which led the hairpin vortex to be broken down into turbulent spots. With the further development of the turbulent spot, the fully developed turbulent boundary layer was achieved. In addition, the strong negative fluctuation of flow velocity was observed in the transition process. The stable standing vortex was formed in the near front region of the semi-sphere, the standing vortex and the induced effect of the hairpin vortex were analyzed in detail. The disturbance created by the small semisphere was spreaded in the form of wedged shape, and the spreading angle was 5.7° for both one and three semi-spheres cases.