One modified poly(ethylene terephthalate) (m-PET) containing 16 mol % of isophthalic acid content and three kinds of isotactic polypropylene (i-PP) with different molecular weights were melt blended in various volume ratios by a twin screw extruder. The dynamic viscosity of the i-PP/m-PET blend showed positive deviation at lower volume content of i-PP and negative deviation at higher volume content of i-PP. The size of dispersed phase increased with an increase of the minor component. Also, the less difference between the viscosities of two polymers caused the smaller particle size in the blend and the smaller critical Weber number. After melt spinning of these binary polymer blends. m-PET and CPP micro staple fiber, whose diameter was about 0.13 to 2.75 mum. could be obtained by extracting the continuous phase with a proper solvent. The less the minor component in the blend, the better the spinnability of the blended polymers and the physical properties of the fiber. Also, when i-PP was the continuous phase, the spinnability and the physical properties of the fibers were better than the opposite case. Molecular weight of i-PP and the blend ratio had a marked influence on the diameter of microfibers. The microfiber obtained from i-PP which has similar viscosity with m-PET showed the smallest diameter than those obtained from other two i-PPs.