HRW said child recruitment rates in the Asian country, formerly known as Burma, remain essentially unchanged compared with four years ago. The agency said up to 20 percent of Myanmar's soldiers are children.
U.N. Secretary-General Kofi Annan has twice reported to the Security Council that the Myanmar government is in violation of international laws.
In 2004, the Myanmar government set up a high-level committee to stop the recruitment of child soldiers, but the practice has not stopped, the agency said.
"Until the government takes genuine steps to implement its laws, children will continue to be snatched off the streets and forced into military service," said Jo Becker with HRW's children's rights division.
The HRW report, citing interviews with former child soldiers, said many of them had been deployed to fight against armed ethnic minority groups. The report said recruiters often apprehended boys at train and bus stations, markets and other public places.
Similar recruitment also was noticed among armed ethnic groups but HRW said the rate is declining.