The effects of triphenyl phosphite (TPP) and bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite (BPDP) on the transesterification and thermal properties of poly(trimethylene terephthalate) (PTT) and poly(ethylene terephthalate) (PET) melt blends were investigated. PTT/PET melt blends containing small amount of TPP or BPDP were prepared by mixing them at $280^{circ}C$ for different times using a twin screw type internal mixer, and their thermal properties were investigated by using a DSC. With addition of phosphite compounds, TPP or BPDP, during the melt blending, the trans esterification reaction was retarded and the melting point depression decreased. TPP was more effective than BPDP in decreasing the transesterification during the melt blending. NMR analysis showed that the randomness of PTT/PET blend containing 1% of TPP prepared at the same melt mixing conditions was much lower than the one without TPP, which confirmed the effectiveness of TPP in suppressing the transesterification reaction of the PTT/PET blends.