Djibril Bashir and Valon Avdylim -- of Somali and Albanian origin, respectively -- are accused of having fought with the IS group in Syria.
Avdylim was also charged along with his younger brother Visar with trying to send military equipment to another sibling who was killed in Syria in April 2014.
The trial marks the first time Norway has invoked a new provision under its criminal law allowing prosecution of any "economic or material support to a terrorist organisation," an offence punishable by a maximum of six years in prison.
But he told the court he was unaware the organisation had been labelled a terror group by the United Nations at the time.
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"I noticed that many other insurgents were very corrupt in the way they handled civilians. Many of them set up checkpoints, kidnapped civilians, demanded money and stole," he said of his time in Syria.
Norwegian authorities feared that the two main defendants -- who returned to Norway in January last year -- would try to return to Syria or commit an attack on Norwegian soil.
According to intelligence services, more than 70 Norwegians have travelled to join jihadist insurgents in Syria and Iraq.