The　whisker　formation　behaviors　of　Sn3.0Ag0.5Cu　composite　solder　which　reinforced　by　an　organic-inorganic　hybrid　materials,　silanol　of　polyhedral　oligomeric　silsesquioxane　(POSS-silanol)　was　investigated.　The　thin　solder　films　were　obtained　by　reflow　soldering　and　a　thermal　cycle　from　-40　degrees　C　to　+85　degrees　C　was　applied　to　accelerate　the　test　process.　Both　surface　and　interface　microstructures　of　the　specimens　were　evaluated　using　scanning　electron　microscopy　(SEM).　The　results　revealed　that　the　growth　of　whiskers　was　inhibited　significantly　in　solders　with　the　addition　of　POSS-silanol.　Fewer　whiskers　with　smaller　size　were　observed　on　the　Sn3.0Ag0.5Cu+POSS　solder　when　compared　with　Sn3.0Ag0.5Cu　solder.　The　main　driving　force　for　the　whisker　formation　under　such　condition　was　considered　to　be　the　squeeze　due　to　the　deformation　of　the　solder　matrix　which　induced　by　the　coefficient　of　thermal　expansion　(CTE)　mismatch.　The　POSS　addition　alleviated　the　deformation　which　dominated　by　CET　mismatch.　Thus,　the　driving　force　of　whisker　formation　was　weaken　through　releasing　of　the　stress　concentration　during　the　iterative　of　heating　and　cooling　process.