I configured nagios core 3.4 in ubuntu 12.04, it's working fine and I installed postfix for mail notification

when am typing in terminal
root@ns:~# mail praveen.ks@xx.com
________________am getting mail____________

but, in nagios am not getting notification. In command.cfg file i given like this

'notify-host-by-email' command definition
define command{
command_name notify-host-by-email
command_line /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\nHost: $HOSTNAME$\nState: $HOSTSTATE$\nAddress: $HOSTADDRESS$\nInfo: $HOSTOUTPUT$\n\nDate/Time: $LONGDATETIME$\n" | /usr/bin/mail -s "** $NOTIFICATIONTYPE$ Host Alert: $HOSTNAME$ is $HOSTSTATE$ **" $CONTACTEMAIL$
}

# 'notify-service-by-email' command definition
define command{
command_name notify-service-by-email
command_line /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\n\nService: $SERVICEDESC$\nHost: $HOSTALIAS$\nAddress: $HOSTADDRESS$\nState: $SERVICESTATE$\n\nDate/Time: $LONGDATETIME$\n\nAdditional Info:\n\n$SERVICEOUTPUT$\n" | /usr/bin/mail -s "** $NOTIFICATIONTYPE$ Service Alert: $HOSTALIAS$/$SERVICEDESC$ is $SERVICESTATE$ **" $CONTACTEMAIL$
}

In contacts.cfg file i given like this

define contact{
contact_name nagiosadmin ; Short name of user
use generic-contact ; Inherit default values from generic-contact template (defined above)
alias Nagios Admin ; Full name of user
email praveen.ks@xx.com ; CHANGE THIS TO YOUR EMAIL ADDRESS ******
}

Can anyone please check and tell me, how to resolve the issue

You first need to make sure that the mail program is actually located at /usr/bin/mail, so type in "which mail" and "which printf". If the paths don't match, modify them.

Then, you need to test the command from a terminal, to see what error(s)/message(s) you get. So, using what you have above:
If that works from the command line, then you have other issues we can help with. If it DOESN'T, then you have identified an issue. And do you see anything in your logs when you try to send an alert?

mail in /usr/bin/mail and printf in /usr/bin/printf only, when am sending mail in terminal it's working, but when host is down am not getting any notification

Right...so did you try pasting the ENTIRE command line onto a terminal prompt as suggested? Did it work??

Yes, it's working

Ok...what exactly did you put in??? Guessing isn't fun...