Hello,

Eligibility criteria is as:

Candidates must have passed 12th class or qualifying exam in 2016 or 2017.

Candidates who are appearing in class 12th or qualifying exam in 2018 can also apply for JEE Main 2018.

If you are a diploma studrnt then you can't apply for jee mains as per the above criteria.

For more details visit the below given link: 

https://engineering.careers360.com/articles/jee-main-eligibility-criteria

Good luck.

Diploma students who have completed their degree, or are in the final year of doing so, are eligible to apply for JEE Main 2018, but will not be given admission to NITs and CFTIs. If they qualify JEE Main, they will have to appear for JEEAdvanced, and they will be given admission like class 12th pass students.

Source: https://www.toppr.com/bytes/jee-main-exam-pattern/

Only those candidates who have passed their Class 12th Exam or any equivalent qualifying examination in 2016 or 2017; or those who are appearing in their Class 12th Exam or any equivalent qualifying examination in 2018 are eligible to appear in JEE(Main)- 2018.