According to the Thai News Agency (TNA) the barter deal will be signed by officials from the Thai defence ministry and the Chinese in Bangkok on 13 December.
The agency did not specify the type or amount of Chinese weaponry that Thailand would receive in exchange for its fruit.
The first shipment of dried longans is expected to head for China before the end of the year, TNA reported.
Longans, which are similar to lychees, are popular across Asia with those produced in Thailand considered by connoisseurs to be among the best.
Thailand's armed forces recently announced plans for a major upgrade and modernisation of its equipment.
Military chiefs have said they envisage spending $6.6 billion over the coming decade, but Prime Minister Thaksin Shinawatra has said barter deals will be given the top priority in procurement deals.
"They both have wings and they can both fly"
Thai prime minister
Thaksin hopes to use barter deals as a way of reducing the country's trade deficit by trading Thailand's abundant farm goods for costly hi-tech military kit.
Last year, during visits to Stockholm and Moscow, Thaksin suggested trading Thai chicken for Russian or Swedish fighter jets despite the air force's preference for US-made F-16s.
"They both have wings and they can both fly," Thaksin was quoted as saying.