Oxfam says deaths in DR of Congo: 100,000 in 3 months - 3.9m over past 8 years - 1,200 people are dying every day from conflict-related causes
Oxfam says donor countries have contributed only $94m (£50m) to a $682m special appeal launched in February.
It says more than 100,000 people have died in the three-month period from diseases that might have been cured.
Oxfam says DR Congo is a forgotten disaster zone in which 3.9m people have died over the past eight years.
The Humanitarian Action Plan was launched by the United Nations, the Red Cross and aid agencies in February.
It comes as the country approaches UN-backed elections in July, which will be the first democratic polls the DRC has ever held.
The BBC's World Affairs Correspondent Mark Doyle says it is unusual for aid agencies to name - and try to shame - specific countries.
But Oxfam has called the contributions of the United States and Japan "minuscule" compared with the size of their economies and said that Germany and France had committed little and Italy nothing.
The aid agency calculates what it calls "fair shares" by comparing the amount appealed for with the size of economies.
According to Oxfam, Britain gave only about half of the share it could have been expected to contribute.
But a spokesman for the Department for International Development told the BBC that the UK had pledged £60m in additional aid to the DR Congo over the next two years to help with the humanitarian situation in the country.
He said the money would go to providing emergency food and shelter, medical equipment , clean water and to rebuild schools destroyed by the conflict.
Oxfam's representative in Congo, Juliette Prodhan, said it was good that donors had agreed to help finance the forthcoming polls, but that the country's problems would not be cured by voting alone.
"Rich country governments have a moral obligation to act when 1,200 people are dying every day from conflict-related causes," she said.
After years of war and misrule, there is little infrastructure in DR Congo, which is about the size of Western Europe, and there are no road or rail links from one side of the country to the other.
Fighting - particularly in the east - continues between rival militias and government forces. Hundreds of thousands of people have been displaced from their homes.
A United Nations peacekeeping force of nearly 17,000 troops - the world's largest - operates in the country and is being augmented by a 1,500-strong European Union rapid reaction force over the election period.