I have a created an ext3 partition and when i tried to set is hidden flag, there seems no effect.
So, how to set the hidden flag of an ext3 partition?
There's no such thing as a "hidden flag" for ext3 partitions. Or for any partition, really; the partition identifier gets changed to something else instead (0x0b -> 0x1b, 0x07 -> 0x17, etc.).