Philosophy of engineering lays the philosophical foundation of recognition, understanding and management of engineering. Being the kernel of philosophy of engineering, engineering ontology becomes the master key to understanding of engineering. The paper proposes and interprets the principal theses of engineering ontology, which differs from understanding of engineering in separate elements. Engineering ontology believes that engineering is the direct, realistic productivity that runs dynamically and feasibly and creates values. Engineering involves the relationship between human beings and the nature as well as the relationship between human beings and the society, and it has been a basic motive force and a basic way of promoting the social development, so that engineering gains the ontological status and fundamental value in social existence and social development. From the historical point of view, the engineering appears before the emergence of technology and science. Engineering has its own basis for existence, its own structure and its own laws for movement and evolution. Engineering should not be simply regarded as the ramification and derivative from science or technology. Engineering ontology is the theoretical basis of the triism of “science, technology, and engineering”. To understand and handle the mutual relationship among engineering, technology and science, by the evaluation criteria of engineering as the direct productivity, the process and effect of engineering-centered selection, integration and construction must be emphasized and the characteristic and mechanism of selection, integration and construction must be paid high attention. Under no circumstance may the engineering be deemed as an unchanged matter, which is constantly evolving and developing, so the studies on engineering ontology are closely and internally related with the theory of engineering evolution.