A man has been sentenced to three years jail for causing the death of a woman when he drove under the influence of drugs.

Lee David Toplass was high on cannabis and amphetamines when he drove along the wrong side of the Kwinana Freeway in July last year.

He crashed head on into a car driven by a 37-year-old woman who died at the scene.

Her 18-year-old son who was a passenger in the car suffered severe injuries.

Toplass pleaded guilty to dangerous driving occasioning death, driving under the influence of drugs and dangerous driving causing bodily harm.

In the Fremantle Magistrates Court today he was jailed two years and 8 months for the woman's death and another 4 months for causing grievous bodily harm to her son.

He will be eligible for parole after serving 18 months.