Land　suitability　evaluation　(LSE)　for　fruit　planting　can　be　used　to　improve　the　survival　rate　of　fruit　tree　planted　in　specific　geographical　conditions,　and　thus　increasing　the　production　and　economic　value　of　fruit　trees.　It　is　a　complex　issue　concerning　many　disciplines　and　also　a　research　relied　heavily　on　expert　knowledge　and　experiences.　In　this　paper,　the　technique　of　ontology-based　knowledge　base　system　is　introduced　for　knowledge　representation,　sharing　and　reuse　in　the　LSE　domain.　An　ontology-based　knowledge　base　system　for　subtropical　fruit　planting　suitability　evaluation　(FPSEOKBS)　is　initially　developed.　This　paper　focuses　on　the　mechanism　of　ontology　based　knowledge　reasoning　and　its　usage　on　LSE.　An　ontology　base,　a　rule　base　and　an　evaluation　unit　map　which　are　essential　for　the　construction　of　FPSE-OKBS　is　expatiated　in　detail.　After　that,　the　incompetence　of　factor　weighting　in　FPSE-OKBS　is　analyzed,　and　it　is　overcome　by　the　employing　of　a　rule　optimization　method　based　on　Analytical　Hierarchy　Process　(AHP).　The　suitability　evaluation　for　typical　a　subtropical　fruit　(lychee)　in　Fujian　coastal　area　is　carried　out,　and　the　effectiveness　and　accuracy　of　the　integrated　application　combining　ontology-based　knowledge　reasoning　and　AHP　technique　in　FPSE-OKBS　is　confirmed.　In　the　discussion　of　evaluation　results　derived　from　different　rule　bases　(rule　bases　before　and　after　rule　optimization),　it　is　found　that　the　evaluation　result　is　much　closer　to　the　real　situation　when　taking　the　AHP　process　into　ontology-based　rule　reasoning.　Furthermore,　through　the　comparative　analysis,　some　limiting　factors　for　typical　subtropical　fruits　planting　in　the　sample　region　of　Fujian　coastal　area　are　obtained:　the　northern　part　of　the　sample　region　is　unsuitable　for　subtropical　fruit　planting　because　of　the　constraint　of　the　local　weather　conditions,　while　the　suitable　condition　of　the　southern　part　is　mainly　affected　by　Soil　Organic　Matter.　Copyright　©　2014　American　Scientific　Publishers.