"The CPC refused the shipment due on October 27 because of suspicion it was oil that had been subjected to US sanctions (against Iran)," the company said in a statement.
Sri Lanka had access to limited quantities of Iranian oil before the US tightened sanctions last December.
Colombo, a net importer of oil, turned to Saudi Arabia and Oman for crude as well as refined product from other countries, including Singapore.
The CPC said with the rejection of the suspected Iranian crude oil, the country would be forced to shut down its refinery until new stocks of crude could be sourced.However, the company said it had enough refined oil to meet local demand.
The US-led sanctions drive is aimed at forcing Tehran to abandon its controversial nuclear programme.
Tehran insists its nuclear programme is only for civilian use, not to make weapons, and refuses to abandon its uranium enrichment activities.
US Secretary of State John Kerry said Friday that world powers and Tehran had yet to strike a deal on Iran's nuclear programme but were working hard to do so.