External dead links are tougher to handle, because they don't occur as a result of anything you've done. You're linking to a page on someone else's web site, and you have no control over that web site. If they take down a page or move it, your link goes bad immediately. I used to have a link on my web site to an article I'd written several years ago for Entrepreneur.com. Eventually, that article expired and they removed it from their web site so my link to it became a broken link. This is a common experience for almost anyone who has external links on their web site.

A little less obvious is the fact that outbound links from your site to other sites are also a factor in your rankings. But that's a little more complex. For example, if you link to a bunch of travel web sites, the search engines will reasonably think your site has something to do with travel (even if it doesn't really relate to travel.) If you link to a number of highly relevant and highly regarded web pages, that can enhance your reputation with the search engines. If you link to web pages with relevant keywords in your link text, that helps, too. But linking to lots of irrelevant or low quality web sites will hurt you.

If you link to pages that just aren't there, it makes your web site less trustworthy. It may imply that you're careless about properly maintaining your site. While many people know that Google measures your link popularity with a PageRank score, many aren't aware that search engines measure your trustworthiness with a TrustRank score. If Google finds a bunch of dead links on your site, can affect your TrustRank and in turn affect how well you rank when people search for what you do.

Another factor to consider is that human-edited directories provide very valuable links back to your website. But the fact that they are human-edited means that your web site is exposed to more scrutiny than it does in non-human-edited directories. If the editor finds a lot of broken links on your website, they may decline to list you. Clearly, it's important to avoid having dead or broken links on your website.

There are services available that will check your links for you and report any dead links they find. They're convenient, reliable, and usually have a reasonable cost associated with them. If you have the time and the discipline to do it yourself, it's not very complicated. Simply go through every page on your site and click on every link that's there. If it doesn't take you where you intend, it's a broken link and you should fix it.

If you have more than a few pages on your site, that process may be more difficult or time-consuming than it sounds. If you find checking for broken links takes more time than you're willing to spend or if you don't remember to do it regularly (monthly is advisable) then you might want to explore having a service do it for you.

Keeping your site clear of broken or dead links means there's one less obstacle to great search engine rankings.This can affect your site/forum rank!

yeah, I agree with you that dead links may broken our SEO site

Thanks for this great information

Dead links are unavoidable. Most sites update their content time to time and delete the old information which were indexed by the search engines. However dead links does add frustration for people using the search engine. Most of the search engine spiders update their links and content algorithm with in a month phasing out the dead links in their index.