Abdullah Khadr, 28, disavowed earlier incriminating statements he made to police. Khadr has been held in Canada on a U.S. warrant since his December 2005 arrest in Toronto.
His father, Ahmed Said Khadr, was an alleged al-Qaida militant and financier, killed by Pakistani forces in 2003.
A brother, Omar Khadr, is the last Western detainee held at Guantanamo Bay. Omar is accused of killing an American soldier with a grenade during a 2002 battle in Afghanistan.
Another brother has acknowledged the family stayed with Osama bin Laden.
The U.S. alleges Abdullah Khadr bought AK-47 and mortar rounds, rocket-propelled grenades and containers of mine components for al-Qaida for use against coalition forces in Afghanistan. He allegedly bought the weapons at the request of his Egyptian-born father, authorities said.
Pakistani intelligence officers detained Abdullah Khadr in Pakistan in October 2004, and he was returned to Canada in 2005. The Royal Canadian Mounted Police arrested him at the request of the U.S.
Authorities said he confessed during questioning in a Pakistani prison, but his lawyers charge that the confessions were obtained with torture.
He told the hearing on Monday that he lied to the Royal Canadian Mounted Police during an interview just after he flew back to Canada four years ago because he feared being sent back to Pakistan, where he had been tortured.
Abdullah Khadr insisted on the witness stand that he knew nothing about his father's dealings with senior al-Qaida leaders and denied buying guns.
"I've never harmed anybody in my life," he told prosecution lawyer Howard Piafsky. "Why would I help people buy weapons?"
"To make money," Piafsky said.
"There are other ways of making money," he responded.
Defense lawyer, Nate Whitling, said the prosecution was trying to portray his client as guilty by association.
The hearing is expected to last three weeks.