This question has been answered in other posts, but I couldn't find the exact one. There are many agile testing "tricks" that are applicable to a traditional waterfall project (assuming that is what your project is). Agile testing is about a mind set, thinking how can I work with the team to understand what the customer really wants or working with the developers to get the best test coverage possible. If you are able give developers your tests before they they start coding, it can save a lot of misunderstandings.
All these things are not limited to agile projects, although the short iterations make is much easier.
Co-author, with Lisa Crispin: Agile Testing: A Practical Guide for Testers and Agile Teams (Addison-Wesley, 2009) www.janetgregory.ca
Is it possible to apply Agile testing process in on on-going project? or can only applied to new projects?
I might be misinterpreting your question, but I think you are asking if agile development works well only in greenfield projects?
It's certainly easier to implement agile development in new projects, but there are many success stories for "legacy" projects switching to agile. There are more cultural and organizational challenges to overcome - it is hard to learn new habits, even if your old habits don't work well for you.
The first two agile teams I worked on did greenfield projects. Everyone came in knowing we would be using XP practices, and everyone was highly motivated to use agile practices and principles. My third team worked on a legacy system, and was never motivated enough to successfully implement agile, or even try very hard to do it. The cultural barriers were just too high.
My current team was in bad shape 5.5 years ago, not able to deliver new functionality to production, the waterfall model wasn't working at all, the code was buggy and poorly tested. The very first sprint where we implemented Scrum, things started to turn around. We've had our struggles over the years, but through diligent use of retrospectives to identify and address problem areas, and good leadership in the team, we've become highly productive and have delighted our customers.
Co-author, with Janet Gregory: Agile Testing: A Practical Guide for Testers and Agile Teams (Addison-Wesley, 2009) http://lisacrispin.com