Jailed: Eakachai Hongkangwan.

Jailed: Eakachai Hongkangwan. Photo: AP

A Thai man was sentenced to more than three years in prison on Thursday for selling copies of an Australian documentary about Thailand's royal family, his lawyer said.

The imprisonment of Eakachai Hongkangwan, 37, is the latest in a series of tough sentences handed down by the kingdom's courts for royal defamation, to the dismay of human rights campaigners.

"The court found him guilty of lese majeste and sentenced him to five years, but due to his useful testimony, the sentence was reduced to three years and four months," lawyer Anon Numpa said afterwards, adding his client would appeal.

Eakachai was also fined 64,000 baht ($A2,117) for selling CDs without a licence.

The documentary was broadcast by the ABC in 2010 in the midst of a military crackdown on anti-government Red Shirt demonstrations in Bangkok.

Thailand warned at the time that the broadcast could affect ties with Australia.

The royal family is an extremely sensitive subject in politically turbulent Thailand. King Bhumibol Adulyadej, 85, is revered by many Thais but has been in hospital since September 2009.

Rights groups have criticised the kingdom's royal insult laws as an attack on freedom of expression.

Under the legislation, anyone convicted of insulting the Thai king, queen, heir or regent faces up to 15 years in prison on each count.

In January the European Union said it was "deeply concerned" by an 11-year jail sentence handed to a former magazine editor in connection with two articles deemed offensive to the royals.

Thailand has been riven by political divisions since the Red Shirts' hero, former premier Thaksin Shinawatra, was topped by royalist generals in a coup in 2006.

Two months of mass street protests by the Red Shirts against the previous government in early 2010 triggered the kingdom's worst civil violence in decades with 90 people killed, mostly in a bloody military crackdown.

AFP