The probe follows complaints by Vietnamese sugar producers that Thai exporters, with subsidy from their government, are dumping their products in Vietnam and causing damage to the local sugar industry, the Trade Remedies Authority of Vietnam said in a statement.
Import of sugar from Thailand increased six times year-on-year in the first eight months of this year to nearly 950,000 tonnes, it said.
Vietnamese producers, represented by six major companies, said the surging imports hit their production, causing it to fall by 33 percent to 800,000 tonnes in the 2019-2020 crop.
They have sought an anti-dumping duty of 37.9 percent.
Vietnam removed import duties on sugar imported from ASEAN countries this year in accordance with the commitments of the ASEAN Trade in Goods Agreement (ATIGA).