Du Anxiang, a 54-year-old Chinese citizen, was convicted by a jury at Northampton Crown Court in central England. He was to be sentenced Thursday.
Du killed lecturer Ding Jifeng, 46, his wife Ge Chui, 47, and their daughters Xing, 18, and Alice, 12, at their home in the town on April 29, 2011.
During the trial, the jury heard how Du "massacred" the Dings, his former business partners, in revenge after he lost a 10-year legal battle over a Chinese herbal medicine business.