This study proposes a unique ontology development methodology, called blended methodology for assessment based competency ontology and presents the step by step developing of ontology for competency certification domain. This blended approach is driven by following combination of TOVE, METHONTOLOGY and YAMO method. The methods confirm the quality of the ontology design, more precisely; reuse and complete. Approach for this blended method was adopted from the YAMO method, namely a top-down approach and a bottom-up approach. Thus, assessment based competency domain, formally defined ontology is designed. In the development of an ontology, a group of people was interviewed to determine the scope and domain boundaries. The finding attained from the results of the ontology evaluation in this study are quite impressive. The evaluation phase was found that 74% of the expert's queries were successfully met. This percentage shows the effectiveness and efficiency of the blended methodology based on ontology have been successfully built This work predict that the existing work will have great implications on the ontology researcher and practitioners. This paper describes a novel ontology development methodology and presented how the methodology can be applied to construct reuse domain ontology.