Abstract: In the OPC specification，there are two types of device read requests：synchronous reads and subscription reads.Subscription reads are more efficient than synchronous reads，because the OPC client does not have to waste time polling the OPC server for changed data and waiting for data to come back from a request.This paper makes a deep analysis on the subscription mechanism of OPC COM-DA and OPC XML-DA.We can implement our projects with OPC COM-DA or OPC XML-DA，according to the requirement.