I want to edit ramdisk in boot.img in cm10.1 's rom.. but I can't unpack it. i use android kitchen but error
See, that's better. Rename the boot.img to "kernel.elf", and unpack it using the menu for Xperia 2012 devices. After that you'll have to unpack the real kernel which uses CPIO.
You can also use this guide, which is useful, and work with any Sony kernel.
