Creating a progress bar using AsyncOperation & UI Images using C# (This a solution which I have already found you are free to use it)

This is to show how to create a simple loading progress using unity AsyncOperation.

Step 1; Create a UI image and name it as Progressbar and set the pivot point of the image to left center and width = 50 , height = 20.
Step 2: Create a new C# script and name it as LoadingBar.
Step 3 ; Copy and paste the below Code.

using UnityEngine;
using System.Collections;
using UnityEngine.UI;
using System;
using System.Collections.Generic;

public class LoadingBar : MonoBehaviour 
    public Image loader;
    RectTransform objectRectTransform;
    AsyncOperation async;
    float tempval;

    IEnumerator Start()
        objectRectTransform = loader.GetComponent<RectTransform>();
        tempval = objectRectTransform.rect.width;

        yield return new WaitForSeconds(0.1f);
        async = Application.LoadLevelAsync("Scene1");//add the next scene name that to be loaded
        yield return async;

  void Update()
            tempval += async.progress;
            loader.rectTransform.sizeDelta = new Vector2(tempval, 20f);
        catch (NullReferenceException ex)
            Debug.Log("Async not loaded!");

Step 4: Create a Empty Game object and drag the LoadingBar script on to it.
Step 5: In inspector add the ProgressBar image to variable loader.
Step 6 : Replace “Scene1” with next scene name that to be loaded in line async = Application.LoadLevelAsync(“Scene1”); on the above code
Step 7; Verify that you have added the current scene and next scene in Build settings.
Step 8: That’s it. You can now play the scene

Thanks mate !