In-situ FT-IR technique is used to investigate the polymerization of propylene oxide with potassium t-butoxide as initiator. The epoxy absorbance at 828 cm-1 is the most useful peaks to monitor the reaction. The absorbance of C-O-C stretching vibration at 1107 cm-1 is very strong and also very useful. Yet, the absorbance of C-H stretching vibration in 3000~2800 cm-1 region is very weak and not reliable for In-situ FT-IR analysis. By monitoring the consumption of propylene oxide, the polymerization rate constant k at 33°C was calculated out and an interesting auto acceleration phenomenon was discovered. Moreover, the chain-transfer process was analyzed by measuring the formation of C=C double bonds, which clarified a former confusion about the course of propylene oxide rearrangement.