Barium strontium titanate (BaxSr1−xTiO3; BST) films with compositions varying from x=0 to 1 were produced on TiN-coated Si substrates by plasma electrolytic oxidation (PEO). Mixed Ba(OH)2 and Sr(OH)2 solutions with different Ba/Sr ratios in a total of 0.5M were used as the electrolytes. PEO was conducted with a potentiostatic mode at an applied voltage of 65 V for 1min. Spark plasma discharges occurred at the working electrodes and lasted for about 10–15s during PEO for all the specimens. Single phase cubic BST films were gained at the [Ba/(Ba+Sr)] ratios above 0.6 while cubic BST accompanying partly anatase TiO2 were present at the [Ba/(Ba+Sr)] ratios below 0.4. Sintered-like porous morphologies with nanolayered structures were obtained. More Sr ions were readily incorporated into BST films at fixed [Ba/(Ba+Sr)] ratios of the electrolytes. On the other hand, more Ba ions in the electrolytes would enhance the ionic conductivity and the crystallinity of the films. The measured dielectric constants and dielectric losses of the BST films were compatible with those reported in the related literature. Reaction routes and formation mechanisms of the BST films have also been proposed.