Well crystallized BLT thin films were deposited by RF magnetron sputtering using a home made target of Aurivillius phase Bi3.25La0.75Ti3O12 (BLT0.75). RF sputtering experiments were performed at room temperature with an argon/oxygen mixture, in a pressure range of 20-50 mTorr. Optimisation of the plasma parameters, namely deposition pressure, RF power and oxygen content in the gas phase, allows obtaining BLT films with a chemical composition close to Bi-3,25La0.75Ti3O12. After ex-situ annealing under oxygen atmosphere at 650 degrees C, BLT deposited on Pt/TiO2/SiO2/Si (multilayer) substrates exhibits well crystallized films with granular morphology. A two step deposition process appeared to be necessary in order to reach satisfying dielectric properties. The effect of the plasma parameters on the chemical composition and electrical properties are presented and discussed.