The Norfolk Ranger was ordered to pay £1,000 by a court that heard it failed to show a current employers' liability compulsory insurance certificate when asked by HSE.
HSE inspector Steven Gill said: "Injuries can stop employees from working for a significant period of time, or even for life. It is vital that they are protected.
"That is why this type of insurance is compulsory. Failing to have it potentially leaves staff doubly disadvantaged in the event of an incident or ill health."
The Norfolk Ranger, based in Mileham, admitted not having the certificate earlier this year. Ian Gibb, who runs the landscaping, fencing and tree-work company, refused to comment. Swaffham magistrates' court fined his firm £700, with £300 costs.
The Employers' Liability Compulsory Insurance Act 1969 insists that all employers insure their staff against injury or disease.
