In this paper, a unified spatio-temporal data model (SCUDM) based on state and change is proposed. A state in spatio temporal database is represented by a ternary tuple about object, space and time. Thus, a change in spatio temporal database is represented by a change over a state, which can be projected to object domain, space domain and time domain separately in order to explicitly represent change over object, space and time. An event in spatio temporal database is a relation of changes about spatio-temporal entities, implying the temporal topology of spatio-temporal entities. SCUDM model can support both representation of state and change and representation of spatial-temporal topology.