A permalink (or permanent link) is a URL that points to a specific blog entry after it has passed from the front page to the archives.Blogger generates this permanent link based on your post title(If the title is in English). For example if your post title is “Hello World”, the blog post’s permalink would be something like http://yourblog.blogspot.com/2012/06/hello-world.html   Till date Blogger never allowed the user to control what the permalink should be but it rather assigned a permalink on it’s own.

create a new post. In the post options you will see a permalink option.

If you select the “Automatic URL” (the default option), then Blogger will generate the permalink on it’s own. If you want to specify a custom URL, opt for the “Custom URL” option and you can enter your custom post permalink over there. It will allow you to enter alphabets, hyphens and underscores. It will just ignore underscores. So I just used a combination of hyphens and alphabets to create my post permalink

The Final Permalink will be something like

http://yourblogaddress/yyyy/mm/the custom URL that you specified

In my case it came to something like

http://myblog.blogspot.com/yyyy/mm/custom-permanent-link.html

Blogger doesn’t allow you to modify the Permalinks of old posts but you can make use of this new feature on your new posts  There seems to be no limit to the length of this Custom URL at the moment but Blogger might impose some length limit in future.