Experimental studies have been made to make clear the effect of tripping wires on the transition in a boundary layer on a circular cylinder in a cross flow. The flow patterns are variable according to the angle φ_R, namely, a contact point of the tangent to the cylinder from the top of the roughness, and to the roughness Reynolds number Re_k, defined by the roughness height and the velocity at the outer edge of the boundary layer. In the range of φ_R < 70°, the roughness has no influence on the transition up to Re_k =620, while, at Re_k≥1220, it is found to has full effect on the transition. In this case, the Strouhal number S increases to 0.30 and the drag coefficient C_D decreases to 0.60. In the range of 70°≤φ_R≤75°, the transition occurs at Re_k = 620. Beyond φ_R =76°, the flow is separated by the roughness, and the values of S and C_D are about 0.175 and 1.55, respectively.