In manufacturing, the descriptions on manufacturing tasks are usually vague, incomplete and inconsistent in industry. The task model and its semantic description method are the guarantee of quality of service (QoS), also the foundation for further study of ontology-based service discovery and matching in manufacturing grid (MG). The definition and types of MG task are 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 and description mode of MG task are proposed. According to the description model of service capacity, the semantic description method for MG tasks is put forward.