The fire broke out around 6am, at the depot in Cliftonhall Road, at the Clifton Industial Estate in Newbridge.
The blaze destroyed two cars, two vans, four mini buses four coaches and two camper vans.
Nobody was injured in the incident.
A Lothian and Borders Police spokesman said: “Although the cost of the damage is not yet known, it will be considerable, and we are appealing for any information that can help us trace those responsible for the fire.
“Anyone who was in the area of the Clifton Industrial Estate in the early hours of today, who noticed any suspicious activity, should contact police immediately.
“Anyone else with any other information that can assist our inquiries should also get in touch.”