Abstract: The paper discusses the properties and structure of the PET/PTT filaments under various tension conditions.As the tension increases,the sonic orientation factor of PET/PTT fiber increases and the Yang′s modulus also increases;the crimp shrinkage rate and the curve elongation rate of PET/PTT fiber decrease exponentially.Moisture thermal treatment of PET/PTT filament under fully relaxed state can impart it the most prominent crimp,but the Yang′s modulus of the fiber will decrease significantly;appropriate small tension is beneficial for both the crimp and the Yang′s modulus.