This paper presents a formal conceptual model for multimedia objects，name-ly，OMCOM(object—oriented multimedia conceptual model)．First，the particularities of the multimedia data modelling are analyzed，the inadequacies of existing multimedia data models are discussed．As a result，the idea is proposed that multimedia data modelling should be divided into two layers：one is the conceptual layer which models the contents and structures of multimedia objects，and the other is the presentation layer which models their presentations including their temporal and spatial relationships．OMCOM is the roodel used to implement the conceptual modelling．Secondly．the formal definitions of the elements in OMCOM are given according to the requirements of multimedia conceptual mod-elling．Finally，object algebra is defined，which is based on the lattice property of OM-COM instances．Object algebra is the system of operations over multimedia objects on the conceptual layer．The formal definitions of object algebra operators and expressions are given．