What's the correct way of writing a higher education title (MSc or M.Sc.) [duplicate]

Possible Duplicate:
Should I write “PhD” or “Ph.D.”?

I see that PhD is written PhD while the undergraduate degree is written B.Sc.

What's the reason for this?

Should I write the Master of Science MSc or M.Sc.?

Is Scott Aaronson's CV inconsistent listing his degrees as "B.Sc." and PhD"

This is strictly style manual stuff. American English generally prefers using periods with abbreviations, and British English generally prefers to omit the periods. Both are "correct", but which one is acceptable is a matter of who is accepting it. It's not grammar or spelling, merely a punctuation convention.

I always omit the periods for academic degrees. If a publisher wants the periods, it has copy editors that can insert them. If it's for your university, check the university's style manual. If there's no manual that you must use, be consistent about including or omitting the periods. It's strictly personal preference unless otherwise indicated, or unless doing it one way or the other causes confusion.