I got 78 percent in CBSE 2014, qualified in JEE Mains 2014(got 140), didn’t get a rank in Advanced 2014 and am writing both Mains and Advanced in 2015.

Is 78 percent sufficient for 2015 IIT admission, as per the new criteria or should I write CBSE improvement exam in 2015, if I should, then what is the criteria for 2015 IIT admission for me?

It has been decided in the meeting that JEE Advanced 2015 Eligibility Criteria will be changed from the existing Top 20 Percentile to Top 20 Percentile or 75 per cent marks in the Class XII board exam, whichever is lower.

Please read the brochure for JEE advanced:

page 26 point number 23 stating the condition of performance in Boards 12^th that “One of the criteria for admission to IITs and ISM is that the candidate should
satisfy at least one of these two criteria:
(1) He/she is within the category-wise top 20 percentile of successfulcandidates in his/her respective Class XII (or equivalent) board examination.
(2) He/she has secured 75% (for GEN or OBC-NCL) or 70% (SC, ST or PwD) ofaggregate marks in the Class XII (or equivalent) board examination.”

That makes it safe and secure for you. Start putting your efforts for JEE advanced. All the Best.