From the studies on polycondensation of cationic dyeable polyester polymer and on their properties in the form of films, the following results were obtained; 1. The modified polyester films increase their moisture regain, decrease their viscosity, lower softening point and improve their antistatic character, according to the amount of modifier added in. 2. The isothermal adsorption curves of Crystal Violet, a cationic dye, on the prepared films were all Langmuir type and, as is estimated, the related combination of dyes and polymers should be a ionic bond. 3. Only from the view point of dyeing properties of the films, reasonable quantity of modifier to be added is about 2 mole percent on the basis of dimethyl terephthalate. 4. Effects of high temperature and carrier on dyeing rate of the modified polyester firm can be said very pronounced, as in the case of normal polyester fibers.