Is it possible for an 18 year old with no high school diploma to be drafted in the MLB draft?

I believe it is, you would be considered a free agent I believe. Problem with this is that any team that offers you money will be able to lowball it because you don't have a diploma, therefore less options.

To be elegible for the amateur draft you have to meet one of the following criteria:
1. GRADUATED from high school.
2. Completed your junior year if you attend a 4-year college.
3. Attended at least 1-year of junior college.
4. Be 21 years old.

Based on this information, which is available at mlb.com, I think if a player has not graduated from high school he couldn't be drafted until he turns 21.

I believe I have heard the following:

A GED is considered the equivilent of a High School Diploma. If he has that, he can be drafted (I am ALMOST positive on this point). I believe I have heard of an example of a HS player that was a hot prospect who took the GED before his senior year in order to be eligible for the draft. I do not know the circumstances surrounding this example, perhaps there were some unusual circumstances.