A housewife in northwest China's Qinghai Province was arrested Sunday for beating her eight-year-old stepson to death, police said Tuesday.

Cai Wenzhen, 37, admitted hitting her stepson with a rolling pin at leat 40 minutes Sunday evening, because she was not satisfied with his schoolwork, said Wang Dong, a police officer in Xining Public Security Bureau.

The boy, Qi Haomin, was a second grader at Konglong Road Primary School in Xining, said Wang.

"She said it happened at 8 p.m., and the child was dead on his bed and we received reports about three hours later," said Wang.

He said police are still investigating the case.

Cai married Qi's  father last year. Their next-door neighbor told Xinhua Tuesday that Cai used to abuse the child.

"Last Friday, she left him standing outside for four hours," said the woman surnamed Zhao. "It wouldn't have ended there had I not found out in time and stopped her."

Zhao said the boy was lively and talkative when his mother was alive. "After his stepmom came, he became much quieter than before."

The boy's father, Qi Zhicai, worked in the rural Guinan County, about 200 km from Xining, and rarely came home.

"I married her hoping she would take good care of my son," said Qi, 41. "I know she sometimes hit the child, but I never expected it could harm him."

Cai is a divorcee and her 12-year-old son from the previous marriage is not in her custody, Qi said. The woman has no job.

The boy's teacher, Shen Juxiang, said he was a good student. "He got full marks in mathematics and Chinese last semester."