Fresnoite　is　a　new　ferroelectric　mineral　which　has　potential　applications　in　acoustic　wave　devices　and　non-linear　optical　devices.　In　this　paper,　fresnoite　thin　films　were　fabricated　by　r.f.　sputtering.　The　experiments　indicated　that　the　working　pressure　influences　significantly　on　the　composition　and　structure　of　the　thin　films.　As　the　pressure　decreases,　the　deficiency　of　Ba　and　the　excess　of　Si　in　the　thin　films　were　controlled.　A　thin　film　with　chemical　stoichiometry　was　obtained　at　the　pressure　of　0.3　Pa.　Besides,　the　decrease　of　the　pressure　improved　the　crystallinity　and　orientation　of　the　thin　films.　These　improvements　on　the　stoichiometry　and　microstructure　of　the　fresnoite　thin　films　are　attributed　to　the　increase　of　the　mean　free　path　and　the　impacting　energy　of　ions　at　low　working　pressure.　(C)　2004　Elsevier　B.V.　All　rights　reserved.