Quantitative analysis method for the calculation of three-dimensional (3D) orientation of Poly(trimethylene terephthalate) (PTT) with polarized FTIR-ATR spectroscopy has been discussed. To experimentally obtain a set of four ATR infrared spectra required for 3D orientation analysis, it is necessary to rotate a sample and/or polarizer by 90degrees upon completion of each measurement of ATR spectrum. Quality of optical contact was demonstrated to be a function of clamping pressure along two beam paths in double-edged parallelogram ATR crystal used in this work. For uniaxially stretched PTT, the band at 1410 cm(-1) was found to be a reference band, the absorbance of which is not affected by orientation state as well as conformation and crystallinity change. By controlling the temperature of sample and ATR crystal, the quality of optical contact could be continuously varied. It was found that the absorbance ratio of two waves (A(TM)/A(TE)) experimentally measured was smaller than the theoretical one unless the sample was in a completely molten state. By multiplying the normalized TM absorbance with the theoretical effective thickness ratio (d(e,TM)/d(e,TE)), three attenuation indices along three orthogonal directions were successfully obtained for uniaxially stretched PTT samples.