Both anhydrous and hydrated heteropolyacid can be used to initiate the tetrahydrofuran polymerization in the presence of ethylene oxide promoter, in regard to the crystallization water of the hydrated one as a part of the extra-added water for controlling the molecular weight. The polyether glycol having number-average molecular weight of 2000 +/- 100 or 1000 +/- 50 was prepared in yield beyond 50% by the process without neutralization or neutralization and washing with water after polymerization, respectively. A modified equation based upon conversion of polymerization and concentrations of molecular weight controller and heteropolyacid with a correction factor for predicting the number-average molecular weight of the product was given. The products were characterized by nuclear magnetic resonance, gel permeation chromatography, and differential scanning calorimetry analyses.