This paper proposes an architecture of a real-time EtherCAT master library called RtEML. The controls EtherCAT slaves under EtherCAT protocol in real-time. It provides a simple programming interface which is useful in developing robot application in C/C++ or C#. To achieve deterministic, hard real-time control in Microsoft Windows environment without additional hardware, INtime is used. Since INtime is designed specifically to take advantage of the powerful capabilities of the x86 processor architecture, the proposed RtEML achieves microseconds of real-time performance.