This article attempts to analytically delineate the ontology of truthfulness in Ayatollah Javadi Amoli's intellectual system. The ontology discussed in this article is ontology in its specific sense in the field of library science (the study of science). That is to say, the concept of truthfulness and its synonyms and their root concepts as well as the type of relationships of this concept with other concepts have been explored and explained from about sixty works of this pious scholar. Consequently, from the viewpoint of Allameh Javadi Amoli, the concept of truthfulness, in addition to its specific semantic application (truthfulness in traditions), was used in a comprehensive sense (creations, necessity and formative contingency, namely, the true entity and nonentity, and noumenon and phenomenon); and in his epistemic geometry, the concept of truthfulness is not considered separately from other concepts; and it is a continuum of links with many concepts such as Truth, formation, angels, human spirit, human nature, religion of truthfulness, liberty and freedom, reason and perfect human beings.