The defect structure of a 350-nm-thick epitaxial Ba0.3Sr0.7TiO3 (BSTO) film grown on (001) LaAlO3 has been investigated using conventional and high-resolution transmission electron microscopy. The dominant defects in the film are edge-type threading dislocations (TDs) with Burgers vectors b5〈100〉and 〈110〉. Pure-screw TDs and partial TDs of mixed character were also observed. A rapid reduction of defect density occurred after the growth of the first 100nm BSTO adjacent to the interface. In the top layer of the film, all TDs with b5〈100〉are perfect while those with b5〈110〉are usually dissociated into two partials with a small separation (a few nanometers). However, in the near-interface layer of the film, many TDs with b5〈100〉are split into two or three partials. A high density of extended stacking faults with displacement vectors of 1 2〈110〉type were observed. The stacking faults are associated with dissociated dislocations and partial half loops. The mechanisms for the generation, dissociation and evolution of the TDs are discussed.