Abstract: Resource monitoring systems play important roles in the grid implementations. The Global Grid Forum has presented the grid monitoring architecture （GMA） to solve these problems. Based on the GMA, the DAG-based two-layer grid monitoring system （DTGMS） is introduced, which makes the DAG the logical base of the DTGMS to describe the dependences among monitored resources. The DTGMS consists of maintaining layer and working layer. The maintaining layer manages the metadata and the working layer, while the working layer does the real monitoring work such as gathering, processing and presenting under the control of the maintaining layer. The monitoring agents in the working layer can be divided into control kernel and external modules for dynamic extensions. The interaction protocols among system components and communication optimizations are also covered. Compared with the GMA, the DTGMS satisfies the grid demands better, and provides more functions with lower system consumption.