In order to explore the micro-torsion mechanism of polarization axis, we studied the microstructure of fused region and taper region of polarization maintaining(PM) fiber devices. Micro crystallizations were observed at the fused region on the fused region and the taper region of polarization maintaining fiber devices with scanning electron microscopy (SEM). The amount and size of the micro crystallizations are increased with decreasing drawing speed and their distribution is asymmetry along the fiber axis. Moreover, micro cracks were observed only at the taper region. The micro cracks became broader and deeper with increasing drawing speed and their distribution is also asymmetry along the fiber axis. The asymmetry of these microstructures is the primary reason that results in micro torsion during the fabrication process of polarization maintaining fiber devices and further deteriorates the optical performance, especially extinction ratio.