This paper presents a novel spatiotemporal ontology based on a mereotopological theory in the context of assembly-oriented design, which integrates assembly sequence planning in the early product design stages. Based on a brief literature review on ontology and existing spatiotemporal ontological models, the authors propose to go beyond by defining their own formal ontology in the domain of assembly-oriented design. The proposed ontology provides formal description of product-process information and information consistency checking through the product lifecycle. Here, the ontology covers the spatial, temporal and spatiotemporal dimensions. The ontology uses OWL language and is implemented in Prot´eg´e. The main objective is to provide a product design description by proactively considering its assembly sequence as early as possible in the product development so as to ensure information and knowledge consistency with preliminary information and later introduce a spatiotemporal reasoning layer.