Unity3d how to make cos 90 equal to 0?

Mathf.Cos(90 * Mathf.Deg2Rad); give me the answer is -4.371139E-08 But i need it to be 0

That's the closest you'll get due to floating point imprecision. Check the answer I wrote here for a more detailed explaination:

http://answers.unity3d.com/questions/26450/mathf-sin-mathf-cos