Members of the 130th (St John) Field Ambulance are being honoured with the rededication of a plaque to them.
The Archbishop of Wales and the Duke of Gloucester, who is the Grand Prior of St John, are among 200 invited guests.
The service, at 14:00 BST, is being held at St John the Baptist in Cardiff city centre.
Welsh assembly's presiding officer Dame Rosemary Butler attended.
The 130th unit was made up of St John Ambulance-trained men recruited from coalfields across south Wales.
The men served at some of the most important battles of World War One and 11 were killed in action or died from wounds.
They provided support at the battle of Mametz Wood on the Somme in July 1916 and at Pilckem Ridge during the Third Battle of Ypres in 1917.
The men won 25 military medals for gallantry, two Distinguished Conduct Medals, two Croix de Guerres and three Military Crosses.
A service to remember Welsh medics who won gallantry medals in World War One is being held in Cardiff.