The accused, 60-year-old Zhao, went to the home of Zan last night and killed his wife, daughter-in-law and grandson with a hammer and a kitchen knife, according to police in Botou city in Hebei Province.
Zhao later telephoned his family and said he have obtained "vengeance" before committing suicide at the site of the murder, state-run news agency Xinhua reported.
Zhao's son was previously married to Zan's daughter and the two families have been embroiled in a divorce dispute, police said, adding that an investigation is underway.