The number of children under five who die annually has fallen from 12 million two decades ago, to 7.6 million.
UNICEF and the World Health Organisation say there are many reasons for the change, including better access to health care and immunisation.
But the agencies warn that more must be done in order to reach the United Nations' development goals on reducing child mortality.
About 21,000 children are still dying every day from preventable causes.
About half of all deaths among under-fives in the world took place in five countries in 2010: India, China, Pakistan, Nigeria and Democratic Republic of Congo.
But child mortality in sub-Saharan Africa is declining twice as fast as it was a decade ago.
The BBC reports Sierra Leone ranks among the top five countries where child mortality has improved in the past decade. The others are Niger, Malawi, Liberia and East Timor.
