Ms Yingluck, the kingdom's first female premier and the sister of former leader Mr Thaksin Shinawatra, was toppled from office by a controversial court ruling shortly before the army staged a coup in May.
She faces impeachment on January 23 by the junta-picked National Legislative Assembly over her administration's populist rice subsidy programme, which funnelled cash to her rural base but cost billions of dollars and inspired protests that felled her government.
Ms Yingluck arrived at heavily-policed Parliament House in central Bangkok accompanied by a handful of her party members.
"There is no position to remove me from as the Constitutional Court has already removed me as prime minister," she told assembly members, also saying she should not be impeached for violating a constitution that no longer exists under junta rule.
Ms Yingluck also defended the rice scheme as an attempt to support Thailand's rural poor, who historically receive a disproportionately small slice of government cash.
"I am not corrupt, I was never careless," she said, urging members to consider her case with fairness and "without being guided by anyone".
A successful impeachment needs three-fifths of the 220-strong assembly to vote in favour.
A guilty verdict would bring an automatic five-year ban from politics and risks enraging her family's 'Red Shirt' supporters, who have laid low since the coup.
Speaking before the hearing January 22, MrJatuporn Prompan, chairman of the Red Shirts, cautioned against street protests on his television show, after noting signs he believed would "lead to impeachment".
"From tomorrow, we will see more clearly... If we are not patient, Red Shirts will be accused of being responsible for bad things," he said on Peace TV.
"It is not over on the 23rd, tomorrow is not the end. Time will tell... We have to be patient," Mr Jatuporn urged his viewers.
by Apilaporn VECHAKIJ
© 1994-2015 Agence France-Presse