Sunil Kumar first gave poison to his 6-year-old son Ganesh and daughter Mahima (3) after which he himself consumed poison at his home, Tohana Deputy Superintendent of Police Shamsher Singh said.
Neighbours rushed the children to a hospital in Tohana where they died, while Sunil breather his last at Agroha Medical College in Hisar district.
The man in his suicide note stated that he was fed up with the marital discord.
"We have registered a case. A team of Forensic Sciences Laboratory has taken samples from the crime spot," he added.