The　thermophoresis　of　nanoparticles　in　the　free　molecule　regime　plays　an　important　role　in　the　nucleation,　collision　and　coagulation　of　nanoparticles.　In　this　contribution,　on　the　basis　of　a　rigorous　gas　kinetic　theory,　the　theoretical　expressions　for　the　thermophoretic　force　on　nanoparticles　suspended　in　dilute　gas　mixtures　are　derived,　wherein　the　non-rigid-body　interactions　between　the　gas　molecules　and　the　nanoparticle　are　taken　into　account.　The　analytical　formulas　in　the　present　work　are　consistent　with　those　under　the　rigid-body　collision　assumption.　Negative　thermophoresis,　is　found　to　be　possible　for　nanoparticles　in　the　free　molecule　regime,　by　which　the　thermophoretic　force　exerted　on　the　suspended　nanoparticle　is　from　low　to　high　temperature,　i.e.,　opposite　to　the　direction　of　normal　thermophoresis.　©　2018　American　Society　of　Mechanical　Engineers　(ASME).　All　right　reserved.