i send one encrypted mail to my friend and when i m going to see what happend i am seeing this

Have you checked in the KMail message menus to see if a "Decrypt" option is available?

hmm ... in this message has nothing else .. maybe a make a mistake

That is what encrypted messages look like.

It would be a security issue if kmail would decrypt it automatically for you.

Check the settings under Configure KMail -> Security Section -> Composing tab. You probably have "Store Sent messages encrypted" which is good for security. If you have the signature of your friend as well as your own on your keyring you should be able to decrypt it automatically within KMail. What you are seeing is what you usually see if gpg is not installed or the keyring isn't readable.

You may find the following pages helpful:

http://userbase.kde.org/KMail/PGP_MIME
http://userbase.kde.org/KMail/gpg

If you haven't already done so, make sure that pinentry-qt4 is installed, and also install kgpg.

If the mail is encrypted with someone's public key, only the one who has the private key can decrypt it.

So, if kmail doesn't do the encryption (I have no clue how it is managed in kmail) with both receiver's and sender's keys and keep both, the sender will not be able to decrypt the message.

That's exactly how it works. You have your private key and your friend's public key. You sign it with the combination of the two - all you have to do to read the message is to provide the passphrase for your own part of the key. If gpg-agent is running you can set it to cache for a specific time, too, so that it doesn't become a nuisance. Since I need my keys regularly I set it to require a password once an hour. UserBase explains the use of all this.

"You sign it with the combination of the two - all you have to do to read the message is to provide the passphrase for your own part of the key."

It is not the matter of signing. If you meant it 'encrypts' the message with both your public and someone else's public key, then ok.

(i'm a bit of cryptology nut )

You are correct. I should have written that you encrypt it with the combination of the two keys. Apologies.