Manufacturing grid (MG) is the application of grid technology in networked manufacturing. Ontology has been considered as an important theoretical basis in many areas such as information integration, semantic web and knowledge management. The descriptions on manufacturing tasks are usually vague, incomplete and inconsistent in industry. MG task model and its semantic description method are the foundation and guarantee of quality of service (QoS) in MG. The MG task model is presented. The combination of ontology with MG study, in terms of analyzing basic classes in MG, and basic ontology classes, requirement model of service capacity of MG task are proposed. According to the description model of service capacity, the semantic description method for MG tasks is put forward.