WASHINGTON (AP) _ Sen. George Mitchell, a member of the Senate panel investigating the Iran- Contra affair, said Thursday that ''evidence of criminality abounds'' and he expects people to go to jail.

Mitchell, D-Maine, said in a CBS television interview that he would be surprised if nobody is on trial or in prison by some time next year.

''Evidence of criminality abounds in this case: obstruction of justice, conspiracy to defraud the United States government, misuse of funds,'' Mitchell said. ''I think we'll see criminal prosecutions almost certainly.''

Mitchell, reacting to President Reagan's news conference, said the president was incorrect when he said he did not believe ''that Iran can give orders'' to the kidnappers of hostages in Lebanon.

''I think the reality is that the kidnapping organization the Hezbollah is financed and controlled directly by the Iranians, and that's why of course two hostages were released immediately after arms were shipped to Iran,'' Mitchell said.