Grid computing has been a hot spot recent year. In grid computing applications, resource management and job scheduling are the most crucial problems in grid computing systems. In this paper, we try to present a novel agent-based dynamic grid resource management model considering both the resource management and job scheduling as a coalition integrity, so as to gain good robust, scalability, efficiency, and high performance. In our model, we use a carefully designed agent to obtain dynamic real time available computational ability of various nodes in the grid environment, so that new job could be assigned to the node that has the largest available computational power, and thus to improve system load ability and performance with good load balance. We construct all grid computational resources into two-layered Heap Sort Tree (HST); this makes the system be more scalable, robust, fault-tolerant and high performance. By taking advantages of agents in constructing and reconstructing the two-layered HST, this model is well fitted with the unpredictable changing grid environment.