Siva,
Ishikawa (a.k.a. Fishbone or Cause & Effect) diagrams can be used as a technique / tool in identifying the root cause of quality related problems or identifying root causes of risk. The Ishikawa technique / tool can be used for both. If I understand this correctly, the root cause analysis is the analytical technique used to determine the root cause and the Ishikawa diagram is a tool that can be used to carry out that effort. Hope this answered your question.
Mark