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.