insert the pendrive in the USB port and wait for it to be detected (or mounted according to Linux)
once it is mounted (ie it opens in a window) , go to the terminal and type the following command :
$ df -h
it will display the list of all mounted drives on your system. It will look something like image shown below :
now very carefully note the path for your pendrive , it would be written in front of the name of your pendrive. Like in above example the data to be noted is /dev/sdb
it might be in your case something else like /dev/sd1
Note: be very sure that you get the correct name otherwise you will end up formatting some other drive
Unmount the USB or Pendrive.
It can be done in two ways :
1. simply right click on pendrive icon on desktop and unmount.
2.form terminal run following command :
$ sudo umount /dev/sdb
i prefer the second method since if pendrive is unmounted after command you can be sure you got the correct drive name in step 1.
Remember : you r only supposed to unmount the pendrive not to remove form the usb port.
Step 3 :
run the following command from the terminal :
$ sudo mkfs.vfat -I /dev/sdb
which will format the pendrive. If you want to provide and label or name to ur pendrive use following :
$ sudo mkfs.vfat -n "name/label"
Note: u can format in any file system u wish to by replacing vfat by that name example ext3 .