Russian news reports say a court ruled Monday that Mikhail Khodorkovsky and his business partner are guilty of embezzling property.
The verdict means the two have been found guilty of stealing oil from Khodorkovsky's now-defunct company Yukos.
The judge has yet to announce a sentence.
Former chief executive of Yukos, Khodorkovsky - once Russia's richest man - and his partner, Platon Lebedev, are accused of embezzling 350 million tons of oil and laundering about $25 billion in proceeds.
Both defendants deny the charges, saying they were framed for opposing Kremlin policies.
Khodorkovsky is currently nearing the end of an 8-year sentence for fraud and tax evasion.