Mirror modifyer not mirroring the movement of an object to be specific im creating a character, i mirrored the left eyeball and i grab it and  move towards the centre which suppose to make the two eyeball closer , but what what happend instead is the right eyeball moved outside the skull. When i move the other objects, such as eyebrow, it is mirroring the movement. Dont know why just the eyeball is moving wrongly, thanks for the help
Sounds like you're in object mode. Check that. It should get closer if you move the eyeball in edit mode.