Laurent Gbagbo and his supporters were accused of wide-scale human rights abuses as they used the military to cling onto power despite losing last November's presidential election to Alassane Ouattara.
Ouattara finally took control of the world's top cocoa grower earlier this month when his forces, backed by French and United Nations soldiers, captured Gbagbo after days of heavy fighting.
"A preliminary investigation has been opened against Laurent Gbagbo, his wife and his entourage," Ivory Coast's new Government spokesman Patrick Achi said on Wednesday refusing to give any further details.
"I cannot say what the main charges are. That is the task of the courts," he added.
The treatment of Gbagbo will test Ouattara's ability to deliver justice as well as reconciliation in a country that was torn apart by an election that had been meant to heal wounds from a 2002-3 war.
Gbagbo and his wife are currently being held in separate locations in northern Ivory Coast, Ouattara's stronghold from where former rebels launched their attack on Abidjan in late March.
The post-election wrangle killed more than 1,500 and forced 1 million people from their homes in Abidjan alone.
The country's cocoa trade and banking sector, which were shut down by the violence and international sanctions aimed at forcing Gbagbo from power peacefully, are set to resume within days.
But Ouattara is still struggling to snuff out resistance from remnants of pro-Gbagbo militia in parts of Abidjan and is also battling divisions within the ranks of the forces that sided with him to oust Gbagbo.