Abstract: This paper studied the effect of alkali treatment on morphological structure and mechanical properties of PTT,PET fiber through SEM,loss of weight,tensile curve and resilient-elasticity recovery.The experiment results show that hightemperature alkali treatment can make surface of PTT fiber dissolve to form a lot of micro-holes,and bring about much more loss of mechanical properties than room-temperature alkali treatment.Alkali resistance of PET fiber is superior to that of PTT fiber in room-temperature,but in high-temperature alkali resistance of PTT fiber is superior to that of PET fiber.Through alkali treatment in appropriate temperature and concentration,PTT fiber can maintain high mechanical properties.