Employment pass application in Singapore

A history of having rejected an invitation to becoome PR will not affect your EP application.

May i know if you have experienced or encountered such situation?

I only have heard and read about such cases, no first hand experience.
You should also be aware that MoM decides on a case-by-case basis, and no two cases are ever the same. Furthermore, they never disclose their reasons for approval or rejection, so everything is guesswork.
But if you were desirable enough for Singapore to invite you to PRship a few years ago, what makes you think you are now so undesirable that you won't even get an EP? What has changed and why?

That's a very good point.
One of the reason I rejected the letter of offer was because the offer expects me to do National Service in return.
So I am assuming that I was 'desirable' then because I could provide service to the NS.
Thank you for input.

Only second generation PRs need to do NS - that means only if you became PR because your parents are PR.
If you are invited on your own merit (as you presumably were), you are exempted from the NS obligation.