Google has no problem with <iframe>s. In fact, no search engine I know of has any problem with them. But as another member pointed out, the content from the document pointed to by the src attribute is not considered a part of the parent page. This can be a good thing because it allows you to use <iframe>s to prevent extraneous material from diluting the focus on the primary keywords for a page. For example, I use <iframe>s embedded within <noscript> tags to warn users running with JavaScript disabled that some features of that site require JavaScript. I also include instructions for turning JavaScript back on. It's useful information for some users, but I don't want that material to detract from the overall theme of the parent page and the <iframe> does that very well. You do have to keep in mind that the search engines (including Google) will index the document included with an <iframe> unless you block it through your robots.txt file or with a robots <meta> tag set to "noindex". So its possible, although very unlikely in most cases, that the <iframe> document may someday appear in the search results and you need to consider that possibility when using <iframe>s.
ok ... thanks for valuable information , but i need to ask one Q. is Adsense dont allow the use of iFrame ...?? thanks ...