I would suggest looking into self hosting your blog, then you’ll be able to run your own ads. Here is another article that clarifies the difference between a self-hosted blog and a blog hosted at WordPress.com: http://en.support.wordpress.com/com-vs-org/
Then what should I do if I need to affiliate my blog with ads company? can I transfer my free wordpress blog to self hosted blog ? because I want to use my domain which I bought through wordpress and can I transfer my current blog to self hosted blog ?