A 15-year-old boy accused of setting fire to a 34-year-old man in Mandurah on the weekend has been granted bail in the Perth Children's Court.

The court heard the boy was at home on Saturday night when a fight broke out between the victim and the boy's father.

It was alleged the boy sprayed a flammable liquid on the man, hoping it would have the same effect as capsicum spray.

The court heard the men continued to fight, moving outside near the front door.

It was then that the boy is alleged to have walked outside with a bottle of flammable liquid and a lighter, spraying the man's upper body before creating a flame torch and setting the man alight.

The man was found nearby a short time later by police who noticed his injuries and breathing difficulties.

He was taken by ambulance to Fiona Stanley Hospital.

The court heard the 34-year-old sustained superficial burns to his face and neck, and more serious burns to his ears.

He also had a deep laceration on his forehead.

The 15-year-old was arrested by police soon after and charged with causing grievous bodily harm.

The boy's lawyer said he intends to plead not guilty, claiming he acted in self defence.

The boy is due to reappear in court on August 20.

Police also arrested a 37-year-old man for allegedly deleting CCTV footage of the incident.

He is due to appear in Mandurah Magistrate's Court on August 21, charged with wilfully destroying evidence.

