North Korea ranked number one on the persecution watchdog's 2008 World Watch List, the seventh year in a row that the communist country has come out on top.
Open Doors and other Christian rights groups say North Korea denies its citizens the right to freedom of worship and report imprisonment and torture of people caught practising the Christian faith.
"It is certainly not a shock that North Korea is No 1 on the list of countries where Christians face the worst persecution," says Carl Moeller, President and CEO of Open Doors USA.
"There is no other country in the world where Christians are persecuted in such a horrible and systematic manner."
No 2 on the list was the Wahhabi kingdom of Saudi Arabia, while Iran came in at No 3. Both countries are ruled by a strict interpretation of Shariah law under which apostasy is punishable by death.
In the last five months, Compass News Direct reported that a daughter of a member of Saudi Arabia's religious police was murdered after she wrote online about her faith in Christ, while a 28-year-old Christian man was allegedly arrested by Saudi authorities for telling of his conversion and criticising the kingdom's judiciary on his website.
Open Doors said 2008 had been one of the toughest years for Christians in Iran since the Islamic Revolution in 1979 with a major crackdown on house churches and a large number of Christians arrested.
Afghanistan moved up three places from last year to become the fourth worst persecutor of Christians in the world, according to Open Doors, which said the Taliban had put greater pressure on Christians during 2008. There was international outcry when Christian aid worker, Gayle Williams, was shot and killed by the Taliban last October as she walked to the offices of the charity she volunteered for in Kabul.
Afghanistan was followed on the World Watch List by Somalia, the Maldives, Yemen, Laos, Eritrea and Uzbekistan.
Somalia moved from No 12 in 2007 to No 5 last year after a sharp increase in reports of persecution against Christians in 2008, while Open Doors said around 3,000 Christians were being held in Eritrean prisons.
India also moved up eight places to No 22 after the second half of 2008 was blighted by the worst outbreak of religious violence against Christians in Indian history, particularly in the state of Orissa, where believers were murdered and their homes and churches burnt down.
"The escalation of violence against Christians in India in 2008 is very troubling," says Moeller. "Please pray for believers there."
Countries to move down the list were China and Bhutan. China moved from No 10 to No 12, while Bhutan moved from No 5 in 2007 to No 11 last year.
Open Doors said it was positive about the drop in persecution against Christians in Vietnam, which moved from No 8 in 2006 to No 23 in 2008, and long-time World Watch List entry Colombia, which has now come off the list altogether.
Open Doors is encouraging Christians to join in its ongoing prayer campaign for North Korea, but especially during North Korea Freedom Week April 25 to May 2.