Thomas Lubanga was convicted by the ICC in March – the first conviction since the court was set up 10 years ago.
He had protested his innocence and said he had not supported the use of child soldiers.
But in a unanimous decision, the judges said Thomas Lubanga was responsible.
Thomas Lubanga showed no emotion as the presiding judge read out the sentence.
Judge Adrian Fulford told the court in The Hague that, taking into account the time Thomas Lubanga has already spent in jail, he will effectively spend eight more years behind bars.
Campaign group Human Rights Watch says more than 60,000 people were killed in the conflict between Hema and Lendu ethnic groups in Ituri, in north-eastern DR Congo.
In June, ICC chief prosecutor Luis Moreno-Ocampo said he was asking for a “severe sentence” of 30 years.
He said the prosecution was requesting a sentence “in the name of each child recruited, in the name of the Ituri region”.
The conviction of Thomas Lubanga is linked to current unrest in DR Congo.
Rebel forces are advancing towards the country’s main eastern city of Goma.
They are headed by General Bosco Ntaganda, who is also wanted for war crimes by the ICC.
Thomas Lubanga
• Leader of the Union of Congolese Patriots (UPC), an ethnic Hema militia
• Head of the UPC’s military wing, the Patriotic Forces for the Liberation of Congo (FPLC)
• Accused of recruiting children under 15 as soldiers
• Arrested in Kinshasa in March 2005
• Held by the ICC at The Hague since 2006
• Born in 1960, has a degree in psychology
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