I think this question is too general to be answered precisely. Cloud like many other technologies has abundant of limitations. In what area are you willing to carry out research? Resource provisioning? performance? usability? VM migration? Reliability, Fault telorance? just to name a few. What do you want to test and why do you want to test? Do you have any solution in mind to work on? 

@Kratzke, @Abolfazli

Thank you for your reply.

Actually i am new in this area. I like to work on testing SaaS on public clouds. Can you suggest me some papers or resources on it?

Can you please be more specific again. Such as if you plan to test the services quality? Availability? Security and privacy level? 

All of these are area of testing , and if you plan to go in security level you can go to security audit , service audit 

I like to mainly focus in security issues of SaaS on public clouds. Since i am new in this can anyone suggest from where i can began?

Thank you

Hi Muhammad, I have recently been involved in cloud computing so I can suggest some online resources that I am aware of. Based on your last message, you may benefit from the links below; the first one is an example of a testing process, and the following two links have a list of possible testing tools to use. Please read these links, and they may help you clarify what you really would like to do. Afterwards, let us know what you have decided, and how your research is going. Thanks! 

Emre 

Dr. E 

http://www.rightscale.com/blog/cloud-management-best-practices/performing-security-testing-cloud

https://sw.csiac.org/databases/url/key/2399/8764/8766

http://www.cloudcomputingexpo.com/node/2625872