I really love her but she always keeps on lying. I asked her not to do so but she carries on. I don’t want to lose her but I hate liars’. She said that this is not a good thing. I also fought with her many times, at that time she says sorry, but then after some days again the same thing. Now I also can’t believe her if there is a tree in front of me and if she’ll say that this is a tree. I don’t want to lose her. Please tell me what I should do.

You either continue to put up with it, or walk away. It depends on how strongly you feel about it.

I have a friend,(just two male Navy friends-not romantic)who has done this (lying), for 50 yrs that I know of, and it hasn't changed one bit.
I have learned to deal with it and we even joke about it.
But, my point is that you shouldn't expect what seems to be a basic trait of some folks, to change. And, expect it to be a lot more difficult to deal with when it is a spouse, than with a friend.

You either continue to put up with it, or walk away. It depends on how strongly you feel about it.
I really love her,but I don't want to leave her.wat should I do??

As Homegirl said - if you won't leave, then you put up with it. This is how it will be your whole life. You don't change people. Sometimes walking away does give the person a reason to change BY THEIR OWN DECISION, and she may work on it just to get you back.
When you are an adult there will be people who drink, do drugs, spend too much, sleep around too much, drive too fast - all sorts of things, and again, you accept it or walk away.

We see more and more young people who can't grasp the notion that they can't have everything they want. You can't have your friend the way you want her to be. End of story!

You may not want to lose her, but you have no choice but to let a liar go. Maybe she will change later, but for now she has not.

Find yourself a new friend. Her lying can cause you much trouble, too, even when you don't know anything about it. We are known by who we hang out with. Or, as the old saying goes, we are known by our associates.
Smile, and others will want to meet you and talk with you. Any good relationship needs honesty, respect, caring, and a willingness to talk about anything. This relationship doesn't have honesty, or respect. Good luck.

fredg brings up an excellent point. Bad traits of a close friend can rub off onto the truthful person. You can overcome it as people experience your honesty but it is a needless hurdle, if it can be avoided and much more damaging when it's a spouse.

If she lies to you, she will lie to others, and it will be no fun defending her lies.

I had a friend who always lied to me, when I was in fourth grade. I actually moved and don't see her anymore.
I never knew she was lying; I on the other hand, was amused with everything she said, I really loved her, even when my older sister told me that I should not talk to her because she is lying to me, I strangely loved her even more. Reverse Psychology.

I never told her to change or that she should stop, I never even brought this topic. I was her only friend. I just looked forward to see her, and listen to her fascinating tells.

Wise people learn when they can; fools learn when they must.

Your friend can't change, unless she has to.