A 31-year-old father, who threw petrol on a man, held up a lighter and threatened to kill him in Perth's south-east, has been sentenced to 16 months in jail.

Brent James Carney's girlfriend had told him the victim had physically and sexually assaulted her, and was also stalking her, so Carney and his brother waited for the man to arrive at her Armadale house on March 30, 2015.

The District Court of WA heard on Friday that Carney attacked him after bursting out of the carport and told the man: "Stay away from Lisa or we'll f***ing kill you."

The man was punched to the head and berated, then ran from the siblings screaming for help, but he was chased, caught and again assaulted.

The man eventually got back to his car and was able to drive off.

Judge Linda Petrusa said she could appreciate Carney wanted to protect his girlfriend but there was no evidence the victim had done anything to her and even if he had, the "vigilante action" was not justified.

Carney, an e-cigarette "juice" trader, will be eligible for parole after serving eight months behind bars.

His brother Dylan was acquitted.

AAP

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