Zn0.9Se:Co-0.1　thin　films　were　deposited　on　sapphire　(Al2O3)　substrates　in　argon　atmosphere　at　various　gas　pressures　by　pulsed　laser　deposition.　Influence　of　argon　pressure　on　the　thickness,　surface　morphology,　crystal　structure　and　optical　properties　of　the　thin　films　were　investigated　by　various　diagnosis　tools.　It　was　found　that　these　physical　properties　were　correlated　to　the　argon　pressure　and　appeared　like　a　mutation　between　the　pressure　of　2　Pa　and　4　Pa.　As　the　deposition　pressure　changed　from　2　Pa　to　4　Pa,　the　collision　probability　between　ablated　species　and　argon　molecule　was　increased,　which　resulted　in　the　transformation　of　the　deposition　type　from　sputter　effect　to　adsorption　effect　and　the　mutation　of　film　thickness　and　structure.　These　in　turn　influence　the　transmission　range　and　band　gap　of　the　films.　All　of　the　results　suggest　that　the　ambient　pressure　is　a　very　important　factor　to　the　deposition　of　Zn0.9Se:Co-0.1　films　by　PLD.