By Kas Chanwanpen
Abhisit won 67,505 votes from Democrat members from across the country, while Warong and Alongkorn received 57,689 and 2,285 votes respectively.
The official results were announced on Saturday.
The Democrats held a primary vote last week, the first time ever that a Thai party has selected its leader with input from its members.
Abhisit won over Warong by 9,816 votes. A total of 127,479 votes were cast.
The result showed the influence of former party secretary-general Suthep Thaugsuban in the country’s oldest political party.
It is widely known that Warong is the nominee of ex-protest leader Suthep, who led a protest against the Yingluck government and showed strong support for junta leader Prayut Chan-o-cha to become the next prime minister.
Abhisit's win could also shatter the dream of the pro-regime Palang Pracharat Party to have an alliance with the Democrat Party in forming the next government to support junta leader Prayut becoming the prime minister.
The Democrat Party will hold its first general assembly on Sunday and will vote on a new executive board as well as a leader. Although primary voting is not a final decision, it is traditional that the voting on Sunday would follow the trend.
Warong, who had been considered a strong candidate against Abhisit, wrote on Facebook that he accepted the results and would work alongside Abhisit for the party as well as the country.