The non-isothermal crystallization kinetics of two thermotropic polyesters with flexible spacers in the main chain were studied. The influence of the spacer type on the kinetics has been considered. The Ozawa theory for nonisothermal crystallization was used to analyse the differential scanning calorimetry data. The polyester with an oxyethylenic group as the flexible spacer followed the Ozawa equation and allowed the calculation of the Avrami exponent under non-isothermal conditions, but in the case of the polyester with methylenic groups as the flexible spacer the Ozawa theory is not valid.