It is always acceptable to ask a professor for a letter of recommendation. It helps to have done well in the class, but you don't need to have any sort of personal relationship with him/her to ask for a recommendation. That is part of what they are there for and what you pay them for.
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It depends on what level of education you are at, and what you need a letter of recommendation for. Most professors do actually notice their students and remember their faces, if not the grades that go with them -- this is generally enough for them to write a generic letter of recommendation.

Even if they do not know you, many professors will still write a generic letter for you if you explain your situation -- obviously, there are occasionally circumstances where a student is very early on into his/her academic career, and have not taken specialized courses or lab courses (the kind where you generally get to know the instructor personally), and professors know this.

If you are looking for a letter of recommendation to get into a school or organization held in high esteem by professors (Ivy League schools or discipline-related honor societies), you may want to get the professor to know you a bit better before you start asking; this will increase not only the odds of the professor being amenable, but also the odds that the professor will write content of high value into the letter.
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Although most professors will not refuse, it does help to give them plenty of time in advance notice before you need your recommendation. It would probably help, though, in terms of quality of the recommendation, if they were familiar with you personally.
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It would make more sense (and result in a better, more personalized letter of recommendation) to ask a professor with whom you had at least interacted in a small group setting (as opposed to an enormous lecture-type class).

However, as stated above, professors are basically in the business of writing these kinds of letters.

I would add, as a personal anecdote, that I had a professor (with whom I actually had a personal relationship and worked with on several projects during undergrad) tell me "No" when I asked for a letter of recommendation for law school. So it's always good to have a backup plan, and, as an above answer recommended, give yourself plenty of time before you need the letter!!
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In some contexts, professors are supposed to write letters for anyone who asks them but there's no guarantee that the letter will be good.
Grades have very little to do with recommendations because professors are looking at other ways to evaluate a student.
If you have actively engaged in your classes with this professor, you went to her/his office hours, and this professor knows your project in relation to which a letter is requested, you can expect a much more appropriate letter than if you rely on grades or "personal" acquaintance.
Source(s):
I've been teaching at eight institutions of higher learning in the United States and Canada.

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Having written several letters of recommendation, I think it is always OK to ask. Make sure you have a relationship with the person you are asking. This should be someone like a professor that was you thesis mentor or a supervisor or college you worked closely with. It also helps the person writing the letter if you can provide a summary of what you want to be recommended for and some suggestion of things they might write about.
Source(s):
Experience

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