Residents of Mrat town in the eastern countryside of Deir al-Zour uncovered the mass grave, in which they found 100 bodies shot dead by the Islamic State (IS) group, Xinhua cited state news agency SANA as saying.
The grave was found in a ditch some seven km from the town, said SANA, adding that some of the bodies were of children.
The IS has committed many massacres throughout the conflict in Syria. In the town of Kshaikeh, east of Deir al-Zour, people have found another mass grave of tens of bodies.
Earlier in the day, SANA said as many as 100 IS militants had been killed over the past 24 hours during battles with Syrian army in Deir al-Zour.
Citing a military source, SANA said the Syrian air force carried out several airstrikes against the IS positions in the town of Hussainyeh in western countryside of Deir al-Zour, completely destroying them.
The Syrian warplanes also struck IS-held areas in the eastern countryside of the oil-rich province near Iraq.
The military campaign in Deir al-Zour is part of a broader offensive the Syrian army has unleashed against militant groups across the country.
Syrian officials said diplomatic efforts to solve the Syrian crisis and countering terrorism are two separate tracks, meaning that nothing will stop the military offensive against terrorist groups in Syria.
The long-running conflict has claimed at least 250,000 lives and driven 11 million from their homes.