AbstractThe LHD (large helical device) data processing system has been designed in order to deal with the huge amount of diagnostics data of 600∼900 MB per 10 s short-pulse experiment. It prepared the first plasma experiment in March 1998. The recent increase of the data mass obliged to adopt the fully distributed system structure which uses multiple data transfer paths in parallel and categorize all of the computer functions into clients and servers. The fundamental element installed for every diagnostic device consists of two kinds of sever computers; the data acquisition PC/Windows NT and the real-time diagnostics control VME/VxWorks. To cope with diversified kinds of device control channels and diagnostics data, the object-oriented methods are utilized wholly for the development of this system. It not only reduces the development burden, but also widen the software portability and flexibility. One hundred Mbps FDDI-based fast networks will re-integrate the distributed server computers so that they can behave as one virtual macro-machine for users. Network methods applied for the LHD data processing system are completely based on the TCP/IP internet technology, and it provides the same accessibility to the remote collaborators as local participants have.