how can i get iit after my dipoma in polytechnic which i done after class12

Hi,

admission to undergraduate programmes of all IITs is taken through the Joint Entrance Examination (Advanced).

So you have to appear in JEE advance exam for getting admission to first year of the undergraduate program. For this you need first to give JEE main exam. After qualifying JEE main exam for JEE advance you will be eligible to appear in JEE advance exam.

There is no provision of lateral entry in IITs to diploma holders in engineering. So after clearing JEE advance you will be admitted to the first year of the degree program.

Hope this helps.