Sometimes ago a friend of mine asked me how to add reply to comments in blogger. In wordpress blogs you can easily reply to comments. A third party script(by Disqus) is available for blogger but some blogger do not like it.
Ishan .... has come with a code that enable replying blogger comments. But that code was not working with most of the templates as all posts disappears. Now we have the one step code for you. So lets add it.
Live DEMO
1. Getting Blog ID
We need ID of the blog where you want to add this feature. To get blog ID go to Layout >> Page Elements. Now check address bar of your web browser. At the end there is blog ID.
e.g. http://www.blogger.com/rearrange?blogID=5143535933889549339
Then Blog ID is
5143535933889549339
2. Installing Code
Go to Layout >> Edit HTML >> Expand Widget Templates and search for
<data:commentPostedByMsg/>
or
<data:comment.timestamp/>
Now Copy and Paste following code
AFTER
<data:commentPostedByMsg/>
or
<data:comment.timestamp/>
<span class='comment-reply'><a expr:href='"https://www.blogger.com/comment.g?blogID=6572093431885631790&postID=" + data:post.id + "&isPopup=true&postBody=%40%3C%61%20%68%72%65%66%3D%22%23" + data:comment.anchorName + "%22%3E" + data:comment.author + "%3C%2F%61%3E#form"' onclick='javascript:window.open(this.href, "bloggerPopup", "toolbar=0,location=0,statusbar=1,menubar=0,scrollbars=yes,width=400,height=450"); return false;'>Add Reply</a></span>
Now change your blog ID value with that of read value in above code. Also you can change text
Add Reply.
You can also add image instead of Add Reply. Just copy and paste any of following code instead of Add Reply in code of step 2.

<img border="0" src="http://1.bp.blogspot.com/_tbHfaj1A058/TA9YyYWSKEI/AAAAAAAALII/L0FAwjfaoBg/s1600/comments+reply+blogger4.JPG" />

<img border="0" src="http://3.bp.blogspot.com/_tbHfaj1A058/TA9ZZh4bDNI/AAAAAAAALIQ/tnQH7_ajs0o/s1600/comments+reply+blogger5.JPG" />
<img border="0" src="http://3.bp.blogspot.com/_tbHfaj1A058/TA9aE_eG95I/AAAAAAAALIg/mC_e5HRkISg/s1600/comments+reply+blogger5.JPG" />
Or You can upload your own image.
Save template and enjoy!
UPDATE :
Blogger introduced "Reply" option to comments and it is called threaded commenting.