I had submitted a college application essay three days ago, but after I submitted it, I felt so uncomfortable with that one, because the essay was so bad, but I had to do that, it was the last day for priority application. Since I could not change anything for my application now, if I send another essay, which I feel more comfortable, to the college application office, is it OK? Will it have good or bad affects to my application?(the college only requires one essay)

And also, for those colleges that do not require essay, will it good for me if I send them my application essay as well?

You need to send the colleges what they ask for, even if each college/university asks for different things.

If you want to find out about sending a second essay to replace your first one, you'll need to call that college's admissions office to find out. It's not a good idea just to send it and hope. It'll probably get lost somewhere!

=)

But how I can tell them why I want to creplace my essay, the reason...

You better be honest.

Yeah, you are right. So can I tell them I want to send another essay that can be better to reflect and let them to know myself?

Yes, you can explain to the college admissions office that you would like to send another essay because you feel that it better reflects who you are and your abilities. Honesty is important in this situation. It is understandable to want to present yourself in the best possible light, especially if you feel that your first essay did not do justice to your qualities and aspirations. Be polite and respectful when communicating with the admissions office and explain your reasons clearly. They will be able to guide you on the next steps and let you know if it is possible to replace your essay. Remember to follow their instructions and guidelines regarding essay submissions.