thnx man ... I used this : if (Input.GetKeyDown(KeyCode.LeftControl)) { if (controller.height == 2) controller.height = 1.8f; else controller.height = 2; } if (Input.GetKeyUp(KeyCode.LeftControl)) { if (controller.height == 1.8f) controller.height = 2; else controller.height = 1.8f;

... but there is another problem... when I crouch then get Up ... the collider will get over terrain so the camera goes under terrain and fall... IDK what should I do!

Not sure what you meant by using get component but you are able to access the character controller through getComponent

try CharacterController controller = GetComponent();

if(Input.GetKeyDown(KeyCode.LeftControl)) { controller.height = 1; }

ok i constructed a cube and attached a character controller to it with a height of 2

and then i attached a script called character that adjusts theheight when left ctrl is pressed the code was:

public class Character : MonoBehaviour {

CharacterController controller;

// Use this for initialization
void Start () {
    controller = GetComponent<CharacterController>();
}

// Update is called once per frame
void Update () {
    if (Input.GetKeyDown(KeyCode.LeftControl))
    {
        if (controller.height == 2)
            controller.height = 1;
        else
            controller.height = 2;
    }
}
}