She achieved the feat in her very first professional fight on August 17, against the more experienced opponent in the 61 kg lightweight category at the Bao'an District Sports Center in Shenzhen, China.
Zhao, a skilled orthodox fighter who's taller than Linh, fell short as the latter combined strength and dexterity to floor her in the very first round.
The Chinese champion stood up at the count of eight and continued to fight. Linh, however, dominated the rest of the proceedings to score a 36-27 victory and secure the WBO Asia-Pacific belt.
Linh, who has a 16-month-old daughter, had won a bronze medal at the Thailand Open International Boxing Tournament last month.
The 26-year-old will represent Vietnam at the World Boxing Championships in Russia in October and the SEA Games 30 in November. She has previously won gold at the SEA Games in 2013.
A successful boxer in Vietnam, Linh has collected titles in all national tournaments from junior to senior levels from the 57kg category to 69kg class. She also became the first Vietnamese female professional boxer, making her debut last Saturday.
Linh moved from her native Lao Cai Province in the northern highlands to Hanoi when she was 13 to pursue a career in boxing. After two years, she was called up to the national youth team and three years later made it to the national senior team.
"It is a great start for female boxing in Vietnam. It will play a key role in encouraging athletes to follow their professional career and make them more confident in upcoming tournaments," said Nguyen Nhu Cuong, who has coached Linh for 13 years.