The retrial for Damir Dokic was held because ambassador Claire Birgin did not testify in person during the original hearing in June. On Thursday, she was again represented by a lawyer.
Dokic's lawyer, Bosiljka Djukic, said the ruling was "illegal ... according to the same scenario," and announced another appeal.
Dokic, 50, was brought under police escort to the court in Ruma, a town about 50 kilometers (30 miles) northwest of Belgrade.
He was found guilty in June of "endangering the security" of Birgin in Belgrade and unlawful possession of weapons, including a hand grenade, which were impounded during a police search of his home.
Dokic was arrested in May after reportedly saying he would blow up Birgin's car if she didn't stop negative articles about him from being published in Australia.
Jelena Dokic had given interviews to Australian media, saying her father had beaten her.
Dokic told Serbian media he would fire a rocket launcher at Birgin's car, but later said the statements were made in anger. He also has admitted having beaten his daughter.
Jelena Dokic, 26, born in the former Yugoslavia, migrated to Australia with her family as a child and represented her adopted country at the 2000 Olympics.
Dokic, a Wimbledon semifinalist in 2000, renounced her Australian ties in 2001 to move back to Serbia, then decided to compete for Australia again in 2006.
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