Prime Minister Yingluck Shinawatra says she took on the additional role as defence minister to foster closer cooperation between the government and the armed forces.
Ms Yingluck yesterday said she decided to take on the defence portfolio because she wanted to expedite decisions relating to the military.
Her holding both positions will allow the government to better support the military, she said.
Ms Yingluck denied an accusation from the Democrat Party that she wanted to interfere with military affairs.
Democrat leader Abhisit Vejjajiva earlier said the government aimed to get two more seats on the Defence Ministry's committee responsible for the annual reshuffle of top military personnel by having Ms Yingluck hold the defence minister post and adding a deputy defence minister in the shake-up.
The government wants to influence the reshuffle, he said.
Ms Yingluck said she has supervised the armed forces for almost two years in her capacity as director of the Internal Security Operations Command.
She said her performance there should prove the government and the armed forces have maintained their independence in carrying out their tasks.
Ms Yingluck said decisions on military affairs previously had to go through numerous channels and procedures before they reached her. Mistakes could occur along the way, which could lead to misunderstandings, she said.
The prime minister added she wanted communication between the government and the armed forces to become faster and more direct.
Deputy Defence Minister Yutthasak Sasiprapa said Ms Yingluck, as defence minister, will have a say in the military reshuffle this year.
"I think it is good for her to take this position, because the chain of command will be shorter," Gen Yutthasak said.
"She will be closer to military units and work will be streamlined, including this year's military reshuffle." However, he insisted the military reshuffle will be free of political influence.
He said Supreme Commander Thanasak Patimaprakorn and army chief Prayuth Chan-ocha were pleased Ms Yingluck took the defence job.