Can I give the exam of IIT JEE after doing polytechnic?

Diploma holders who have have completed 3 year engineering diploma from recognized Polytechnic are eligible to appear in JEE to seek admission in IITs. Such candidates who seek admission in IITs have to appear in JEE Main but their rank and score for JEE (Main) is not declared as diploma holder

Yes, you can give the exam for IIT JEE after doing Polytechnic. Yes, when you passed your Diploma. You can appear for the IIT JEE exam and be selected with the same process as the 12th students. I hope it helps.