Principal not permitted leave?

Convince him, show him the rule book.

HE IS NOT LISTENING N NOT PERMITTING. IF I RELEAVE WITHOUT INTIMATING ANY PROBLEM OCCURE I MEAN ON COMPLETION HE MAY ENTER MORETHAN LEAVE I HAD TAKEN LIKE THAT.

Your articleship completion form has to be signed by him and so you can not just leave him in a hurry. Try working out some mechanism where you assure him of serving the extra days, in case you take excess leave.
 

If the relation between you both has gone sour, try to reconcile and make him understand what is the sticking point, if you think that is possible.

The last resort is to write to the Institute, which should in all circumstances be avoided, this is my personal opinion, you should not go to that extent.  There can not be anything in this world which can not be sorted out by discussion. The end result is a possible comprise at both ends.

Wish you good luck and hope you are able to convince your Principal and get a positive solution at the end of the day.