The purpose of this paper is to investigate the effect of a semi-circuler tripping wire on the aerodynamic character of a circuler cylinder. An experimental investigation on the fluid forces acting on a circuler cylinder with a semi-circuler tripping wire were carried out. It was found that there were the range of the bi-stable flows in which the short time-mean values of fluid force change intermittently, the mean values of drag and lift coefficient with setting angle of tripping wire from front stagnation point changed remarkably and the strongest correlation was observed between fluctuation of drag and lift.