PurposeNet is a semantic network of artifacts and actions related to artifacts. Since man tends to classify objects and artifacts around him in terms of some primary purpose attributed to them, purpose is taken as an organizing principle for the knowledge base. Actions play a pivotal role in describing artifacts. All processes that describe an artifact, namely, its creation, utilization, maintenance, repair and so on are describable through actions. Therefore, a schema of ‘action semantics’ has been made a constituent part of PurposeNet. This paper presents the architecture of action ontology and significance of the design. Action ontology has three major components: a) semantic roles which describe the action; b) ‘precondition’, a set of criteria to be fulfilled for any action to take place and ‘post-condition’, effect of the action; c) sub-actions, a set of actions which when performed in some order - sequential or parallel lead to the action being executed.