Abstract: In response to the problems that have arisen regarding the suitable organization structure which allows intelligent agents for efficient cooperation and communication reduction, several approaches are suggested to address these problems such as contract net protocol （CNP）, middle-agent, acquaintance model, referral web, etc However, these systems cannot easily scale up due to imposing infrastructure, protocol and communication overheads In this paper a relation web model is proposed for efficient cooperation in a large-scale open multi-agent system （OMAS） In relation web, the acquaintance chains play a big part in transferring and allocating the tasks When facing a task beyond an agent’s capabilities, the agent directly sends part/all of the task to the best qualified acquaintance from its relation web Trust is considered to be important whenever agents must interact in agent sociality, and the propagation of trust-degree is introduced in acquaintance chain Besides, the approach is simple and scales up with no infrastructure or protocol overheads, thus it may be very useful for large scale OMAS