The report reflects the heavy reliance on coal that is hampering China's efforts to tackle climate change.
The country's 10 largest power companies supply nearly 60 percent of China's energy. They burned a fifth of all of China's coal in 2008 and emitted about 1.
44 billion tons of carbon dioxide, which contributes to global warming, the report said.
The three biggest companies China Huaneng Group, China Datang Corp. and China Guodian Corp. produced more greenhouse gases than Britain in 2008, according to the report, titled "Polluting Power: Ranking China's Biggest Power Companies."
All three state-owned companies have also been warned in recent years by China's environmental protection agency as having "severe violations" of environmental safeguards.
While China's per capita emissions remain far below those of developed countries, China as a whole has already surpassed the United States to become the world's biggest emitter of greenhouse gases. The two countries account for 40 percent of the world's total emissions.
"China's power companies are not only the key coal consumer but also the major carbon dioxide emitter," Yang Ailun, Greenpeace China climate campaign manager, said in the report.
Greenpeace China urged China to impose on power companies an energy tax and an environmental tax on coal. It also urged China to double its target of getting its power from renewable energy to 30 percent by 2020.
China has made some progress. Over the past 3 1/2 years, it closed down inefficient coal-fired plants that produced 54.07 gigawatts of energy, equal to Australia's total power capacity, the report said.
Both China and the U.S. are looking for ways to cooperate on addressing the issue of climate change in preparation for UN-led talks in Copenhagen at the end of the year, which will seek to forge a framework for a global climate change treaty.