1.Downlaod LILI (Linux Live) software from internet which is free.
2.Install and run the software .
3.Choose the USB flash drive in which you want to create bootable OS.
4.Select the OS type from which you want to create bootable OS (iso image/cd drive/download)
5.Check the format option and let the USB format completely.
6.Click the lightning icon in the software to start and wait for 10 minutes ,your bootable USB for Ubuntu will be ready.!!!!!