La substituted barium hexaferrite Ba1-yFe12LayO19 with 0≤ y ≤0.5 was synthesized by chemical co-precipitation method using aqueous solutions of metallic chlorides at ambient temperature. NaOH was used as a precipitant. The co-precipitated products were annealed at 900 and 1000°C. The effects of La addition on the phase composition, morphology and thermal behavior of samples were studied using XRD, SEM and DTA/TGA, respectively. XRD results indicated that barium hexaferrite forms together with some intermediate phases in samples annealed at 900°C and the amount of intermediate phases decreased significantly by increasing annealing temperature to 1000°C. XRD results also showed that in sample annealed at 1000°C, the crystallite size of magnetic phase decreases from 98 to 64 nm by increasing the amount of La. Thermal analysis (DTA/TGA) implies that formation temperature of barium hexaferrite is increased with addition of La.