The　goal　of　an　on-line　ontology-based　question-answering　system　is　to　automatically　derive　answers　from　ontology　knowledge　bases　without　demanding　additional　information　or　intervention　from　users.　This　paper　focuses　on　solving　mapping　task　from　question　variable　to　OWL　elements,　which　belongs　to　component　of　question　understanding　in　question　answering　system.　The　paper　presents　a　principled　approach,　which　builds　on　tokens　of　question　and　syntax　or　semantic　relations　from　NLP　into　set　of　variables　and　functions　of　constrain,　and　a　sound　optimization-based　assigning　mechanism　to　find　sound　substitute　in　OWL　knowledge　for　question　variables.　A　preliminary　evaluation　on　the　International　WIC　Institute　knowledge　and　questions　is　presented.