some time i am unable to do pendrive formate im my computer. Wshat should i do when i will fall on this problem. can any one help me please?

What errors do you get? Does your system allow you to access the pendrive? These devices are generally factory formatted with a FAT file system which can be read by Windows, Linux, or Apple system. If you cannot access it, then it is likely a problem with your system software. I used to get this all the time with Windows 7 and had to remove all the USB devices from the device manager, and then reboot the system so it would reload the drivers and such. This was a regular PITA that I, and others, had to go through.

If your uh, USB has a letter (let's assume it has letter B). Use following commands:

DISKPART
list disk
select disk X
format fs=dat32 label="My pendrive" quick
assign letter Z

Don't just copy this, just copy-paste this LINE BY LINE. After list disk, you will be given all the disks connected to computer, replace X with your "disk-number" which in this case is an USB. After this, your pendrive should be
MY PENDRIVE (Z:), if it won't succeed, we hear it from you.

Warning: Be careful with this. And you will be just fine.