No, a diploma holder cannot appear for the IIT entrance examination. But if you have completed your HSC in Science, then you are eligible irrespective of whether you have a diploma or not. If you have passed your HSC in Science with Physics, Chemistry, Mathematics and English, then you are eligible to appear for the entrance examination. If you in SYJC and expect to complete HSC in the year in which you seek admission to IIT, then also you can appear for the entrance examination. IIT-JEE is conducted in two stages now IIT-JEE Preliminary examination and IIT-JEE Final examination. To appear in the IIT-JEE Final examination, you have to qualify in the preliminary examination. Notification of the preliminary examination is published all the leading English daily newspapers in India in the month of July/August. The examination is held in December/January. The final examination is held in May. For applying to the preliminary examination, you have to follow the procedure stated in the notification.