Daniel Bekele worked for ActionAid, an anti-poverty group, while Netsanet Demissie ran the Organisation for Social Justice in Ethiopia.
The two were the last defendants out of 131 originally charged after demonstrators took to the street to protest polls they said were rigged.
A parliamentary inquiry said 30,000 people were arrested.
The government denied election fraud.
Most of those originally charged were freed earlier in the year after the government published a letter it said opposition leaders had signed admitting their guilt.
Rights groups condemned the arrests as an attempt by the government to squelch the opposition because of gains it made in the polls, which had been considered Ethiopia's freest.
Both the activists sentenced on Wednesday were involved in deploying observers at polling stations in and around the capital Addis Ababa at the time of the election.
In announcing the sentence - considerably less than the 10-year maximum - the court said it had "taken into consideration their efforts to help the 2005 elections be conducted in a fair manner and to help the ongoing process of democracy to succeed".
ActionAid said it would appeal against the verdict in an African Union court given the Ethiopian court's acknowledgement the two had worked to aid democracy in the country.
Julian Filochowski, an ActionAid representative, said: "Praise by the court on the performances of the two activists during the election shows that what they had done was legitimate. It's a vote of confidence."