Depends what your question really relates to...
If you mean fear in class - such as doing a big break fall you need to have trust in your instructor. I used to think "all I have to do is try and hold on, the rest will just happen". You have to put faith in the class and if you are practicing with someone really rough just tell them to be easier on you.
If you mean fear of confrontation in a fight then that is different. The best technique for this is to imagine that someone you love is also with you - i.e. your partner, parents, children, niece etc. We all hear stories of finding extra strength and courage when helping someone else yet can't seem to do it when protecting ourselves. So, to 'train' your mind (which doesn't happen enough in martial arts) I'd advise to imagine how you would feel if a lunatic attacked you when you had to protect a loved one - this will make it easier to switch in and out of this phase when needed and to eradicate any fear.