With 94% of ballots counted, 61.4% voted in favor and 38.6% voted against the draft constitution, which could give extra powers to the military
Thailand's military seized control of the government in a 2014 coup.
The referendum Sunday also asked Thais to vote on whether to allow the country's Senate to jointly vote for prime minister along with the House of Representatives.
About 58% voted to allow the Senate to jointly vote for the prime minister, while almost 42% rejected it, according to the Thai Election Commission.
An estimated 50 million eligible voters went to the polls on Sunday, according to the commission.
Official results from Sunday's voting will be declared Wednesday.
A controversial vote
Some voters saw the draft constitution as the legitimization of the military's rule, rather than part of a road back to full democracy, as Prime Minister Prayut Chan-o-cha, former commander in chief of the Thai army, claims.
Chaiwat Rattanawitthayapol, a 46-year-old businessman, told CNN after he cast his vote that he chose not to accept this draft constitution because it was written by the regime. He said he could not bear to see an appointed -- as opposed to elected -- prime minister continue to run the country.
"I can't accept it. It is the military's constitution -- they would just want to write it in the way to maintain their power," he said.
Jirattha Vitthaya, a 40-year-old company worker, said she voted against the new draft.
"This constitution is not from the people ... (those) who wrote it were selected by the military," she said. "They said it is for the sake of the country, but how can only a (small) group of people say this is for the whole country?"
Others said the relative calm that military rule has brought to the country was reason enough to vote in the suggested changes.
Im Jeepetch, 18, a first-time voter, admitted she didn't know much about the draft constitution but voted "yes" because she feels that Bangkok has become more orderly and peaceful since the military came to power.
The calm in the city has made it easier for her to travel to her school, the university student said.
Analysts say the constitution will allow the military to maintain power in future without the need to stage coups, of which Thailand has seen 12 since 1932.
"The draft constitution aims to entrench the power of the military and traditional elite, and if it passes the referendum Thailand will move toward semi-authoritarianism guided by the military," according to the Institute of Southeast Asian Studies, a Singapore-based research organization.
"With the army's power embedded in the constitution, backed by the Constitutional Court's 'state of exception' powers, the traditional elites would in effect have designed new mechanisms to seize power without resorting to a military coup."