Ultrahigh-Q micro spherical cavities with micron dimension diameter is obtained by melting the tip of a fused silica fiber with two counter-propagating CO2 laser beams. And the two counter-propagating light beams optical system is designed and realized. To acquire the optimal condition, the full cavity-formed condition, the ablation process is investigated in real time by a monitor system self-designed. The geometrical figures of built cavities and the surface topography are checked and recorded by an optical microscope and an atomic force microscope (AFM). The results show that perfect silica micro spheres with diameters of 30~ 300 μm are acquired.