The Criminal Court on Thursday sentenced red-shirt co-leader Jatuporn Prompan to six months in jail, suspended for two years, and a fine of 50,000 baht after finding him guilty of defaming Democrat Party leader Abhisit Vejjajiva.
The lawsuit was filed by Mr Abhisit.
It stated that Jatuporn, in speeches from the rally stage of the United Front for Democracy against Dictatorship from Jan 29-Feb 15, 2010, accused Mr Abhisit, who was then prime minister, of ordering soldiers to kill people.
The court convicted and sentenced him.
The court dismissed another defamation lawsuit brought against Jatuporn by Mr Abhisit, who objected to Jatuporn calling him a military draft dodger.
The court ruled that Mr Jatuporn, in making the comment, was only doing his duty as an MP in scrutinising Mr Abhisit's qualifications to serve as prime minister.