The ICC's prosecutor has said political leaders were behind some violence
The International Criminal Court has authorised a formal investigation into the violence after Kenya's 2007 poll.
Some 1,300 people died and tens of thousands were displaced as political differences snowballed into weeks of ethnic score-settling after the poll.
The ICC judges said that crimes against humanity may have been committed.
ICC chief prosecutor Luis Moreno-Ocampo requested the investigation last November, saying political leaders organised and financed some violence.
The ICC authorised the investigation in a majority ruling on Wednesday.
"The information available provides a reasonable basis to believe that crimes against humanity have been committed on Kenyan territory," said two of the three judges.
Aid groups and Western governments have urged Kenya to introduce electoral reform, eradicate corruption and punish those who led the killing.
List of suspects
Many Kenyans will welcome this decision as it is widely felt that unless some people are punished for the post-election violence of two years ago, the events could all too easily be repeated, says the BBC's Will Ross in Nairobi.
So far nobody has been held to account for the events which took Kenya to the brink of civil war, he says
By Will Ross
BBC News, Nairobi
There has been virtually no attempt whatsoever by the politicians to ensure perpetrators of the violence are brought to book.
Much to the disgust of many Kenyans, the politicians are busy positioning themselves ahead of the 2012 polls instead of fixing what went wrong last time.
The ripples created by the ICC are likely to increase dramatically once Luis Moreno-Ocampo releases the names of those he intends to prosecute.
Kenya's coalition government is already shaky and will be severely tested.
But now cabinet ministers and other powerful Kenyans look set to appear in the dock before ICC judges, our correspondent adds.
Kenyan Justice Minister Mutula Kilonzo said he welcomed the move and would co-operate with the investigation.
He told the BBC the ICC was better equipped to take on the investigations than the current Kenyan legal system.
"What the government couldn't do, was to pass a law establishing a local mechanism that would have been able to facilitate investigations locally under best international practice," he told the BBC's Focus on Africa programme.
"That is what we have failed to do."
Following the election violence, international mediators led by former UN chief Kofi Annan were brought in to resolve the crisis.
They brokered a power-sharing government, but repeated attempts by mediators to coax the government into setting up a tribunal to investigate the violence came to nothing.
Last July Mr Annan handed to the ICC a list of those suspected of orchestrating the violence.
The names on the list have not been made public, but correspondents say they are likely to be prominent politicians and businessmen.
Some supporters of indicted politicians are likely to rally behind their leaders, raising the prospect of a new upsurge in political and ethnic tension, our correspondent says.