Set work item so that only owner can modify the work item

This is configured through permissions at the role level. In Eclipse is under Process Configuration -> Team Configuration -> Permissions -> Work Items -> Save Work Items -> Modify Work Items.

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Gabriel Enriquez, IBM Rational, Tracking & Planning

Using the role based permissions under process configuration would let you restrict work item changes to e.g. project managers. But then nobody without that role could modify work items, even if they are owner.
I read your question in the way that you want project managers AND work item owners be allowed to modify work items. If that is the case you need to a server side plugin for a process precondition for Work Item --> Work Item Save (Server), attached to role everybody. I do not think one of the out of the box plugins for Work Item save does exactly what you describe. So you would have to write one.
The logic of your plugin would be:
if (currentUser == owner) or (currentUser.role == project manager) then return ALLOW;
else return FORBID;

However, if you restrict work item modification that way, you are essentially restricting commenting and collaboration on these work items. The precondition probably should be restricted to certain work item types or certain fields of a work item.

- Arne