In the process that uses terephthalic acid(TPA) and 1,4-butandio(BD) for the production of polybutylene terephthalate(PBT), the influences of reaction conditions on % reaction, Xn(the degree of polymerization of oligo-PBT), and %BDlongrightarrowTHF(tetrahydrofuran) were investigated. With increasing mole ratio of BD:TPA, %BDlongrightarrowTHF increased and Xn decreased, but with increasing tetrabutyltitanate(TBT) concentration, %BDlongrightarrowTHF decreased and Xn increased. %BDlongrightarrowTHF increased greatly with increasing the reaction temperature in the rang of temperature between 21$0^{circ}C$ and 23$0^{circ}C$ and the increase of temperature afforded great effect to Xn in the range of temperature up to 21$0^{circ}C$. Drop-wise feed of BD was an effective method for the minimization of THF formation. Assuming that the reactions occurring in both the formation of oligo-PBT from BD/TPA and of THF from BD would be of the first order with respect to H2O and THF respectively, the kinetic reaction constants -KH2O and KTHF-were determined from the results which had been examined in the temperature range between 19$0^{circ}C$ and 21$0^{circ}C$. KH2O /KTHF values were plotted versus the reaction temperature, and the temparature-raising-condition on reaction time for minimal loss of BD by hydration to THF was described.