The optical gap of fluorinated amorphous carbon (a-C:F) films, which were prepared by electron cyclotron resonance plasma chemical vapor deposition using trifluromethane (CHF 3 ) and benzene (C 6 H 6 ) as the source gases, is investigated. The optical gap E g of the a-C:F films, which were deposited at input microwave power of 140-700 W, pressure of 0.1-1.0 Pa and CHF 3 /C 6 H 6 flow ratio of 1:1-10:1, is in the range of 1.76-3.98 eV and is dependent on the amount of fluorine and C C bonding in the films. The optical gap E g increases with raising the amount of fluorine but decreases as the amount of C C bonding increased. This is because of the decrease in the amount of π bonding formed by C C and the increase in that of σ bonding formed by C F. As a result, the density of state near band edgy reduces and the optical gap becomes wide.