The phase transitions from nematic mesophase to isotropic state (N-I) and from isotropic state to nematic mesophase (I-N) of the poly (P-phenylene benzobisthiazole)-methyl sulfonic acid solution (PBZT/MSA) have been studied by depolarized light intensity and polarized microscopy.It was found that the phase transition temperature is a function of theconcentration of PBZT/MSA. The atructural long range order of PBZT/MSA induced by shearing can been enhanced by increasing temperature.The study of kinetics of I-N phase transition shows that the transition is nucleation con-trolled which can be described by an Avrami equation. The Avrami exponent is about l which suggests the nucleation is followed by rodlike growth.The results show that the depolarized light intensity is a simple and an effective method for study of the process and the kinetcs of phase transitions of liquid crystalline polymers

es-pecially for lyotropic liquid crystalline polymers.