I have created a iOS project and want to create class diagram. And before that manually I just wanted to know if there are any UML class diagram generator tool for xcode or any other tool that generates the class diagram using the xcode project.
OmniGraffle is the right tool that exactly matches your need. I have used it a couple of times for generating class diagrams from Xcode projects(using the .xcodeproj file) but I had to do few changes since my design was not top notch then but if the classes are perfectly designed you can get the best out of this tool. Its not free but you get a 30 day trial and after installing go to
File->Open->Browse and select your .xcodeproj file.
Possible duplicate here.