Input.mousePosition

Description

The current mouse position in pixel coordinates. (Read Only)

The bottom-left of the screen or window is at (0, 0). The top-right of the screen or window is at (Screen.width, Screen.height).

Note: Input.mousePosition reports the position of the mouse even when it is not inside the Game View, such as when Cursor.lockState is set to CursorLockMode.None. When running in windowed mode with an unconfined cursor, position values smaller than 0 or greater than the screen dimensions (Screen.width,Screen.height) indicate that the mouse cursor is outside of the game window.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public GameObject particle;
    void Update()
    {
        if (Input.GetButtonDown("Fire1"))
        {
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
            if (Physics.Raycast(ray))
                Instantiate(particle, transform.position, transform.rotation);
        }
    }
}