Yushchenko claims that Tymoshenko was driven by political gain when signing a natural gas import contract with Russia in 2009. Prosecutors say she violated legal procedures in signing it.
Yushchenko charged Wednesday that the price Tymoshenko agreed to for gas from Russia was much too high and can be explained "only by political motives."
He says Tymoshenko, who was preparing to run in presidential elections the following year, wanted to score points when signing the deal ending a bitter gas dispute that led to energy shortages across Europe.
The West has condemned the court case as selective prosecution.