Finally I fixed the issue by following steps.
1) Added the folder that contains the HTML file as "Reference" by selecting "Create folder references for any added folders"
in the XCode
.
2) I used the following code to load the HTML and it worked:
// 360 is the folder which contains the HTML files
NSURL *url = [NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"index" ofType:@"html" inDirectory:@"360"]];
[self.webview loadRequest:[NSURLRequest requestWithURL:url]];