I want to show keyboard when mail composer open without take any action.i mean when mail composer open automatically keyboard should display.
This isn't possible as far as I know. You might be able to traverse the views of the mailComposer to find the appropriate text field but that is a risky thing to do and liable to break.
Plus it might get your app rejected. Although it's not exactly clear. From the MFMailComposeViewController Class Reference:
Important: The mail composition interface itself is not customizable and must not be modified by your application. In addition, after presenting the interface, your application is not allowed to make further changes to the email content. The user may still edit the content using the interface, but programmatic changes are ignored. Thus, you must set the values of content fields before presenting the interface.